iio: adc: mt6370: Add MediaTek MT6370 support

MediaTek MT6370 is a SubPMIC consisting of a single cell battery charger
with ADC monitoring, RGB LEDs, dual channel flashlight, WLED backlight
driver, display bias voltage supply, one general purpose LDO, and the
USB Type-C & PD controller complies with the latest USB Type-C and PD
standards.

Add support for the MT6370 ADC driver for system monitoring, including
charger current, voltage, and temperature.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Link: https://lore.kernel.org/r/81ec58ae89030e48508d6810396de2679c40d26c.1665488982.git.chiaen_wu@richtek.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>