dt-bindings: iio: adc: Add AD7606B ADC documentation

Documentation for AD7606B Analog to Digital Converter and software
mode was added.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>