iio: adc: Add driver for Texas Instruments ADS131E0x ADC family

The ADS131E0x are a family of multichannel, simultaneous sampling,
24-bit, delta-sigma, analog-to-digital converters (ADCs) with a
built-in programmable gain amplifier (PGA), internal reference
and an onboard oscillator.

Datasheet: https://www.ti.com/lit/ds/symlink/ads131e08.pdf


Signed-off-by: Tomislav Denis <tomislav.denis@avl.com>
Link: https://lore.kernel.org/r/20210202084107.3260-2-tomislav.denis@avl.com


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