soc: mediatek: Add MediaTek SCPSYS power domains

The System Control Processor System (SCPSYS) has several power management
related tasks in the system. This driver implements support to handle
the different power domains supported in order to meet high performance
and low power requirements.

Co-developed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20201030113622.201188-3-enric.balletbo@collabora.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>