iio: adc: mt6360: Add ADC driver for MT6360

Add MT6360 ADC driver including Charger Current, Voltage, and
Temperature.

Signed-off-by: Gene Chen <gene_chen@richtek.com>
Link: https://lore.kernel.org/r/1603181267-15610-4-git-send-email-gene.chen.richtek@gmail.com


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