regulator: mt6380: Add support for MT6380

The MT6380 is a regulator found those boards with MediaTek MT7622 SoC
It is connected as a slave to the SoC using MediaTek PMIC wrapper which
is the common interface connecting with Mediatek made various PMICs.

Signed-off-by: Chenglin Xu <chenglin.xu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>