regulator: mt6366: Add support for MT6366 regulator

The MT6366 is a regulator found on boards based on MediaTek MT8186 and
probably other SoCs. It is a so called pmic and connects as a slave to
SoC using SPI, wrapped inside the pmic-wrapper.

Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220317030402.24894-2-johnson.wang@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>