soc: mediatek: add SCPSYS power domain driver for MediaTek MT7622 SoC

Add SCPSYS power domain driver for MT7622 SoC having four power domains
which are respectively ETHSYS for Ethernet including embedded switch,
WBSYS for WIFI and Bluetooth, HIF0SYS for PCI-E and SATA, and HIF1SYS for
USB. Those functions could be selectively powered gated when the
corresponding function is no longer to use in order to reach more minimal
power dissipation.

Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>