soc: mediatek: add SCPSYS power domain driver for MediaTek MT7623A SoC

Add SCPSYS power domain driver for MT7623A SoC. The MT7623A's power
domains are the subset of MT7623 SoC's ones. As MT7623 SoC has full
features whereas MT7623A is being designed just for router applications.
Thus, MT7623A doesn't include those power domains multimedia function
belongs to. In order to avoid certain errors undoubtedly happening at
registering those power domains on MT7623A SoC using the existing MT7623
SCPSYS driver, it's required to define another setup specifically for
MT7623A SoC.

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>