dt-bindings: iio: adc: ad7291: add binding

Add device-tree binding for ADI AD7291 ADC.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200922144422.542669-2-michael.auchter@ni.com


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