Third set of new device support, features and cleanups for IIO in the 4.11 cycle

This also involves a merge of the ib-mid-iio-pwm-4.11 branch from mfd to
bring in support for the stm32 timer triggers needed for the buffered
features in the stm32 adc driver.

New device support:
* Amligic Meson SAR ADC
  - new driver and bindings
* cros_ec barometer
  - new driver
* max5481 digital potentiometers
  - new driver for 5481, 5482, 5483 and 5484
* Renesas GyroADC - a very specific spi offload engine for ADCs
  - new driver and bindings.
* srf08 ultrasonic ranger
  - new driver, bindings and ABI docs,

New features
* Qualcomm PM8xxx ADC bindings.
  - due to a trivial build issues the driver will be following shortly.
* stm32 ADC
  - Triggered buffer mode
  - Allow use of stm32 timer triggers
  - Add trigger polarity control
  - Optional DMA support with bindings update
* stx104
  - add support for gpio names
  - support set_multiple callback
* tmp007
  - optional interrupt support

Cleanups
* ad7150
  - alignment fix.
* ad7816
  - octal rather than symbolic permissions.
* lsm6dsx
  - allow selection of data ready pin via device tree bindings.
* ssp_sensors
  - use devm_iio_device_register to handle unregister automatically.
* stx104
  - use devm functions in probe allowing removal or the remove function.
  - drop unneeded struct stx104_dev
* tmp007
  - fix the name attribute to be a meaninful description of the part.
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