soc: mediatek: avoid using fixed spm power status defines

Use variables to replace fixed defines since the offset
of the status of spm power might be different for some chips

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Kevin-CW Chen <kevin-cw.chen@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>