dt-bindings: iio: adc: add bindings doc for AD9467 ADC

This change adds the binding doc for the AD9467 ADC.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>