dt-bindings: iio: adc: add adi,ad7780.yaml binding

This patch adds a YAML binding for the Analog Devices AD7780/1 and
AD7170/1 analog-to-digital converters.

Signed-off-by: Renato Lui Geh <renatogeh@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>