soc: mediatek: Enable mmsys driver by default if Mediatek arch is selected

The mmsys driver supports only MT8173 device for now, but like other system
controllers is an important piece for other Mediatek devices. Actually
it depends on the mt8173 clock specific driver but that dependency is
not real as it can build without the clock driver. Instead of depends on
a specific model, make the driver depends on the generic ARCH_MEDIATEK and
enable by default so other Mediatek devices can start using it without
flood the Kconfig.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Hsin-Yi Wang <hsinyi@chromium.org>
Link: https://lore.kernel.org/r/20200401201736.2980433-1-enric.balletbo@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>