Second set containing features and cleanup for IIO in the 4.16 cycle

Lots of work on the ina2xx driver this time including some nice tricks
to reduce bus usage.

There are a couple of fixes in here that would have gone via a
faster route if we were ealier in the cycle.

Features
* Core
  - Add a sysfs attribtue to expose the amount of data available in the
    buffer (where known).
* hx711
  - Add triggered buffer support.

Cleanup / Fixes
* Core
  - Trigger core documentation fixes.
* Docs
  - The ABI docs had wrong name for current_timestamp_clock file.
  - Fix some kernel-doc issues
* ad7152
  - BIT macro conversions.
* adis16136
  - Switch to DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe.
* adis16480
  - Switch to DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe.
* bfin-timer
  - replace device_attr with device_attr_rw.
* ad7192
  - Drop unnecessary parentheses
* am335x_adc
  - remove a pointless bool conversion.
* at91-adc
  - Fix a brace warning.
* bmg160
  - drop a duplicate include
* bmp280
  - Read calibration data once at probe, it doesn't change.
* ccs811
  - Scale value was not conforming with ABI.  Would have gone via
    fixes path if we were earlier in the cycle.
* dummy_evgen
  - Modify the check on return from irq_sim_init as first step in a
    rework of that code so that it returns the base of the range on
    success rather than just 0.  Rework will occur in next cycle.
* hts211
  - Move common code from i2c and spi probes to the core one.
* hx711
  - fix a bug in reset that meant two close together reads would not
    actually cause two reads.
* ina2xx
  - Make the calibration register value fixed as there is no actual
    benefit in changing it and it can be a bad idea.
  - Use a monotonic clock for delay calculation.
  - Align the timetsmp with the conversion ready flag.
  - Manipulate the loop timing to make it line up with real timing
    saving on pointless reads.
  - Drop a bogus cast
  - Clarify size required for the data buffer (documentation and code
    adjusted to make the relationship obvious).
  - Remove unnecessary read to clear CNVR flag.
  - Udelay was potentially for several seconds, switch to usleep_range.
* lsm6dsx
  - Fix an endianness bug in *read_oneshot (note this is in this pull
    as, given time in cycle this is probably fastest merge route)
  - Protect configuration with it's own lock.
  - Switch to using regmap to avoid a locally rolled version of
    same thing.  Follow up patch dropped a left over forward
    declaration.
  - Preallocate the read buffer to fix sparse warning on variable
    length array.
* mcp4725
  - Drop pointless bool conversion.
* meson-saradc
  - Fix a slight error in the clocks for Meson8 and Meson8b.  No
    visible result of being wrong though.
  - Make sure the channel muxes are initialized.  Some devices have
    these already programmed, but on a few devices they aren't so
    you can get garbage on unconfigured channels.
* mma8452
  - SPDX license
* ssp_sensors
  - Use ktime_get_real_ns instead of getnstimeofday to avoid 2038
    issues.
  - As of_device_id.data is const, we can make the value which is
    assigned to it const as well.
* st uvis25
  - Make soem structures static
* stm32
  - SPDX license
  - Fix a bug with multiple channel DMA (would have gone more
    directly if we weren't so late in the cycle).
* tsl2x7x
  - Break some long lines.
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