First set of IIO new device support, features and cleanups in the 4.19 cycle

The usual mixed bunch.  Particular good to see is the generic
touch screen driver.  Will be interesting to see if this works
for other ADCs without major changes.

Core features
* Channel types
  - New position relative channel type primarily for touch screen
    sensors to feed the generic touchscreen driver.

New device support
* ad5586
  - Add support for the AD5311R DAC.

* Generic touch screen driver as an IIO consumer.
  - Note this is in input, but due to dependencies is coming through
    the IIO tree.
  - Specific support for this added to the at91-sama5d2 ADC.
  - Various necessary DT bindings added.

Staging Drops
* ADIS16060 gyro
  - A device with a very odd interface that was never cleanly supported.
    It's now very difficult to get, so unlikely it'll ever be fixed up.

Cleanups and minor features and fixes
* core
  - Fix y2038 timestamp issues now the core support is in place.
* 104-quad-8
  - Provide some defines for magic numbers to help readability.
  - Fix an off by one error in register selection
* ad7606
  - Put in a missing function parameter name in a prototype.
* adis16023
  - Use generic sign_extend function rather than local version.
* adis16240
  - Use generic sign_extend funciton rather than local version.
* at91-sama5d2
  - Drop dependency on HAS_DMA now this is handled elsewhere.  Will
    improve build test coverage.
  - Add oversampling ratio control.  Note there is a minor ABI change
    here to increase the apparent depth to 14 bits so as to allow
    for transparent provision of different oversampling ratios that
    drop the actual bit depth to 13 or 12 bits.
* hx711
  - Add a MAINTAINERS entry for this device.
* inv_mpu6050
  - Replace the timestamp fifo 'special' code with generic timestamp
    handling.
  - Switch to using local store of timestamp divider rather than rate
    as that is more helpful for accurate time measurement.
  - Fix an unaligned access that didn't seem to be causing any trouble.
  - Use the fifo overflow bit to track the overflow status rather than
    a software counter.
  - New timestamping mechanism to deal with missed sample interrupts.
* stm32-adc
  - Drop HAS_DMA build dependency.
* sun4i-gpadc
  - Select REGMAP_IRQ a very rarely hit build issue fix.
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