iio: adc: Add basic support for AD4170-4

The AD4170-4 is a multichannel, low noise, 24-bit precision sigma-delta
analog to digital converter. The AD4170-4 design offers a flexible data
acquisition solution with crosspoint multiplexed analog inputs,
configurable ADC voltage reference inputs, ultra-low noise integrated PGA,
digital filtering, wide range of configurable output data rates, internal
oscillator and temperature sensor, four GPIOs, and integrated features for
interfacing with load cell weigh scales, RTD, and thermocouple sensors.

Add basic support for the AD4170-4 ADC with the following features:
- Single-shot read.
- Analog front end PGA configuration.
- Differential and pseudo-differential input configuration.

Signed-off-by: Ana-Maria Cusco <ana-maria.cusco@analog.com>
Co-developed-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://patch.msgid.link/ce3fd150bd63a2aed6eb6fe59aad6d60c0f9fb67.1751895245.git.marcelo.schmitt@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>