mfd: mt6370: Add MediaTek MT6370 support

This adds support for the MediaTek MT6370 SubPMIC. 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.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: ChiaEn Wu <chiaen_wu@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220805070610.3516-8-peterwu.pub@gmail.com