Second set of new device support, features and cleanup for IIO in the 4.18 cycle

Usual mixture of new devices support and other stuff.  A couple of
staging graduations in here and some old driver drops.

New device support
* ad5686
  - Add support for AD5691R, AD5692R, AD5693 and AD5693R i2c DACs
  - Add support for AD5681R, AD5682R, AD5683 and AD5683R SPI DACs
* lmp91000
  - Add ID for LMP91002
* stm32-dfsdm
  - Add support for the stm32mp1 devices.

Drivers dropped
* ADE7753
  - No longer easily available, no users came forward and needs a lot
    of work to move out of staging.
* ADE7754
* ADE7758
* ADE7759

Staging graduations
- ad2s1200 - good cleanup from David Veenstra.
- tsl2772 (was tsl2x7x) - Brian's quest is at an end and
  staging/iio/light is no more!

Features
* tools
  - loop forever on a negative number of loops being specified.
* ltc 2632
  - add of_match_table
  - support an external reference regulator.
* mag3110
  - Support continuous mode when running fast as it increases the
    posssible sampling rate.
* ti-ads8688
  - Add trigger and buffer support to this ADC.

Cleanups / minor tweaks.
* tools
  - tidy up types in iio_generic_buffer.
* ad2s1200
  - Whitespace cleanup.
  - Drop pointless initializations.
  - Improve kernel docs.
  - Reorder to setup the SPI prior to device registration (race fix).
  - Change to modern gpiod framework.
  - Drop platform data and move to DT bindings. There are no in
    kernel users of the platform data. Any out of tree boards will
    need changes.
  - Add an explicit compatible table.
  - Provide _scale for angular velocity and angle channels.
  - Add David Veenstra to copyright notice as this cleanup was non
    trivial.
* ade8366
  - Avoid a race by ensuring channel init is before registration
    with the subsystem.
* afe
  - binding spelling fix.
* imx7d-adc
  - typo fix in Freescale
* inv_mpu6050
  - tidy up an ugly loop to take advantage of known entry condition.
  - add a comment explaining why the sensible sampling frequencies
    are more limited than might be immediately apparent (aliasing).
* mma8452
  - reduce the sleep time on data not ready when running at high
    frequency.
* stm32-dfsdm
  - add missing header.
* tsl2583
  - fix integration_time_availabe which was in microsecs rather the
    millisecs.
* tsl2x7x/tsl2772
  - Whitespace cleanup.
  - Change to direct returns where sensible.
  - Turn the chip off in the registration error path.
  - Use macro to reduce repition when setting up the device_info
    structures.
  - Change the _available attributes over to read_avail callback,
    and the range definitions that supports.
  - Fix some wrong period values.
  - Add some range checkign for _write_raw.
  - Rename the driver to tsl2772 to avoid wild card clash problems
    in future.
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