bindings: iio: adc: Add documentation for ADS131E0x ADC driver

Add a device tree binding documentation for Texas Instruments
ADS131E0x ADC family driver.

Signed-off-by: Tomislav Denis <tomislav.denis@avl.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210202084107.3260-3-tomislav.denis@avl.com


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