regulator: mt6323: Add support for MT6323 regulator

The MT6323 is a regulator found on boards based on MediaTek MT7623 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: Chen Zhong <chen.zhong@mediatek.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Mark Brown <broonie@kernel.org>