mfd: Add support for the MediaTek MT6397 PMIC

This adds support for the MediaTek MT6397 PMIC. This is a
multifunction device with the following sub modules:

- Regulator
- RTC
- Audio codec
- GPIO
- Clock

It is interfaced to the host controller using SPI interface by a proprietary
hardware called PMIC wrapper or pwrap. MT6397 MFD is a child device of the
pwrap.

Signed-off-by: Flora Fu, MediaTek
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>