Second set of new device support, features and cleanup for IIO in the 5.1 cycle.

There are a few late breaking fixes in here that weren't worth trying to
rush into 5.0 as they have been with us for quite a while.

New device support
* ad7476
  - add support for TI ADS786X parts that are compatible with this Analog
    Devices driver. Good to see some simple devices are so similar.
* Ingenic jz47xx SoC ADCs
  - new driver and bindings
* Plantower PMS7003 partical sensor
  - new driver and bindings including vendor prefix.
* TI DAC7612
  - new driver and bindings for this dual DAC.

New features
* ad7768-1
  - Sampling frequency control
* bmi160
  - Data ready trigger support, including open-drain dt binding.

Cleanup / minor fixes.
* Analog Device DACs
  - Fix some inconsistent licenses.  These are only ones where there were
    two different license marked in the same file, and hence were previously
    unclear.
* ads124s08
  - Spelling fix.
* adxl345
  - Parameter alignement tidy up.
* bmi160
  - SPDX
  - correct a note on the types of supported interrupts which was too strict.
  - use iio_pollfunc_store_time to grab an earlier timestamp.
  - use if (ret) instead of if (ret < 0) to be consistent whilst simplifying
    some handling where ret was effectively getting written to 0 even though
    it was always already 0.
* exynos_adc
  - Fix a null pointer dereference on unbind.
  - Fix number of channels on Exynos4x12 devices to be 4 rather than 8.
* lpc32xx-adc
  - Move DT bindings doc out of staging. Oops, I missed this one when
    moving the driver.
  - SPDX.
* npcm-adc
  - drop documentation of reset node as going to be done differently.
    It's a new driver this cycle so no need to support the previous
    binding going forwards.
* sps30
  - Fix an issue with a loop timeout test that meant it would never identify
    a timeout.
  - Mark deliberate switch fall throughs.
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