dt-bindings: iio: adc: Migrate AD7606 documentation to yaml

The documentation for ad7606 was migrated to yaml.

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>