soc: mediatek: Refine scpsys to support multiple platform

Refine scpsys driver common code to support multiple SoC / platform.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>