Documentation: ABI: testing: mt6360: Add ADC sysfs guideline

Add ABI documentation for mt6360 ADC sysfs interfaces.

Signed-off-by: Gene Chen <gene_chen@richtek.com>
Link: https://lore.kernel.org/r/1603181267-15610-3-git-send-email-gene.chen.richtek@gmail.com


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