mfd: Add support for the MediaTek MT6366 PMIC

This adds support for the MediaTek MT6366 PMIC. This is a
multifunction device with the following sub modules:

- Regulator
- RTC
- Codec
- Interrupt

It is interfaced to the host controller using SPI interface
by a proprietary hardware called PMIC wrapper or pwrap.
MT6366 MFD is a child device of the pwrap.

Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220106065407.16036-2-johnson.wang@mediatek.com