regulator: mt6359: Add support for MT6359 regulator

The MT6359 is a regulator found on boards based on MediaTek MT6779 and
probably other SoCs. It is a so called pmic and connects as a slave to
SoC using SPI, wrapped inside the pmic-wrapper.

Signed-off-by: Wen Su <wen.su@mediatek.com>
Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>