soc: Mediatek: Enable SCPSYS power domain driver by default

If enable Mediatek 8173 SoC, it should also enable power domain
driver. Otherwise access clk subsystem register will fail.

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>