dt-bindings: iio: adc: add bindings doc for MT6360 ADC

This change adds the binding doc for the MT6360 ADC.

Signed-off-by: Gene Chen <gene_chen@richtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1603181267-15610-2-git-send-email-gene.chen.richtek@gmail.com


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