mfd: Add support for the MediaTek MT6358 PMIC

This adds support for the MediaTek MT6358 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.
MT6358 MFD is a child device of the pwrap.

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>