First set of new device support, features and cleanups for IIO in the 4.21 cycle

Along with the headline feature of 5 new drivers, we have the
substantial addition of auxilliary sensor support on the lsm6sdx
parts for ST.  There has also been a good set of staging cleanup
in this period with more underway.

An ever increasing number of devices supported with just a new
ID which is a good sign that at least some manufacturers are
continuing to stabilise their interfaces.

New device support,
* ad7124
  - New driver supporting Analog Devices' ad7124-4 and ad7124-8 parts
    with the inevitable DT binding.
* ad7949
  - New driver supporting Analog Devices' ad7949, AD7682 and AD7689 ADCs.
* rm3100
  - New driver supporting PNIs RM3100 magnometer with bindings and
    vendor prefix.
* ti-dac7311
  - New driver supporting DAC7311, DAC6311 and DAC5311 TI DACs, with
    DT bindings.
* vcnl5035
  - New driver supporting the light sensor part of the VCNL4035, with
    DT bindings

Features,
* bindings
  - Add a generic ADC channel binding as we keep reinventing this
    wheel.
* adc128s052
  - Add IDs for additional pin compatible parts.
  - Add APCI ID seen on E3940 UP squared boards.
* ad_sigma_delta
  - Allow for custom data register overiding default.
* kxcjk1013
  - Add KIOX0009 ACPI ID as seen on the Acer One 10.
* lsm6dsx
  - Rework leading to...
  - External sensor support using the built in I2C master.
  - Initial support for a slave lis2mdl magnetometer.
* meson-saradc
  - Add temperature sensor support and bindings.
* st_magn
  - New ID for lsm9dsl_magn with bindings
  - New ID for lis3de accelerometer
* tpl0102
  - Add supprot for IIO_AVAIL_RANGE to report the range available
    from this device to userspace and in kernel users.

Cleanups and minor fixes
* tools
  - Allow outside specification of CFLAGS
* ad2s90
  - Handle and spi_read error.
  - Handle spi_setup failure
  - Drop a pointless assignment.
  - Prevent a potentail race by moving device registration to after
    all other setup.
  - Add missing scale attribute.
  - Add a sanity check on channel type before trying to read it.
* ad2s1210
  - Move to modern gpio descriptors.
  - Drop a gpioin flag which made no sense as far as we can tell.
  - Add dt table (bindings doc to follow when this is ready for
    moving out of staging).
* ad5933
  - Drop camel-case naming of ext_clk_hz.
  - White space fixes.
* ad7150
  - Local variable to shorten overly long line.
  - Alignment and line break fixes.
* ad7280a
  - Handle an error path that was previously ignored.
  - Use crc8.h to build the crc table replacing custom code.
  - Avoid unecessary cast.
  - Power down the device if an error happens in probe
  - Use devm routines to simplify probe and remove.
* ad7606
  - Alignment fixes.
* ad7780
  - This worked as long as by coincidence an uninitialized value
    was 0.  Lets not rely on that.
  - Ensure gain update is only used with the ad778x chips that
    actually support it.
  - Tidy up pattern mask generation.
  - Read regulator when scale is requested (which should be infrequent)
    as it might have changed from initialization.
* ad7816
  - Move to modern gpio descriptors
  - Don't use a busy_pin for ad7818 as there isn't one.
  - Ensure RD/WR and CONVST pins are outputs (previously they
    were brought up as inputs which doesn't seem to make any sense)
  - DT id table.
* adc128s052
  - SPDX
* adt7316
  - Alignment fix.
  - Fix data reading.  When using I2C the driver never actually
    used the value read.  This has been broken a very long time
    hence no rush to fix it now + the driver is undergoing a lot
    of cleanup.
  - Sanity check that the i2c read didn't fail to actually read
    anything.
* dpot-dac
  - Mark a switch full through with slightly different text so that
    gcc doesn't warn on it.
* gyro-adc
  - Fix a wrong file in the MAINTAINERS entry and add binding doc to the
    listed files.
* ina2xx
  - Add some early returns to clarify error paths in switch.
* lsm6dsx
  - MAINTAINERS entry.
* max11100
  - SPDX
* max9611
  - SPDX
* mcp4131
  - use of_device_get_match_data in preference to spi_get_device_id
    approach.
* rcar-adc
  - SPDX
* sc27xx
  - Add ADC conversion timeout support to avoid possible fault.
* ssp_sensors
  - Don't free managed resources manually.
* st-magn
  - Add a comment to avoid future confusion over when to use -magn
    postfix (on multi chip in package parts)
  - Add BDU register for LIS3MDL where it seems to have been missed.
* st-sensors
  - Minor spelling, grammar etc fixes.
* tpl0102
  - Use a pointer rather than an index of an array to improve conciseness.
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