First round of new devices, features and cleanups for IIO in the 4.17 cycle.

Outside of IIO
* Strongly typed 64bit int_sqrt function needed by the mlx90632

New device support
* adc081s
  - New driver supporting adc081s, adc101s and adc121s TI ADCs.
* ad5272
  - New driver supproting the ad5272 and ad5274 ADI digital potentiometers
    with DT bindings.
* axp20x_adc
  - support the AXP813 ADC - includes rework patches to prepare for this.
* mlx90632
  - New driver with dt bindings for this IR temperature sensor.

Features
* axp20x_adc
  - Add DT bindings and probing.
* dht11
  - The sensor has a wider range than advertised in the datasheet - support it.
* st_lsm6dsx
  - Add hardware timestamp su9pport.

Cleanups
* ABI docs
  - Update email contact for Matt Ranostay
* SPDX changes
  - Matt Ranostay has moved his drivers over to SPDX.  Currently we are making
    this an author choice in IIO.
* ad7192
  - Disable burnout current on misconfiguration.  No actually effect as
    they simply won't work otherwise.
* ad7476
  - Drop a license definition that was replicating information in SPDX tag.
* ade7758
  - Expand buf_lock to cover both buffer and state protection allowing
    unintented uses of mlock in the core to be removed.
* ade7759
  - Align parameters to opening parenthesis.
* at91_adc
  - Depend on sysfs instead of selecting it - for try wide consistency.
* ccs811
  - trivial naming type for a define.
* ep93xx
  - Drop a redundant return as a result checking platform_get_resource.
* hts221
  - Regmap conversion which simplifies the driver somewhat.
  - Clean up some restricted endian cast warnings.
  - Drop a trailing whitespace from a comment
  - Drop an unnecessary get_unaligned by changing to the right 16bit data type.
* ms5611
  - Fix coding style in the probe function (whitespace)
* st_accel
  - Use strlcpy instead of strncpy to avoid potentially truncating a string.
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