clk / soc: mediatek: Move mt8173 MMSYS to platform driver

There is no strong reason for this to use CLK_OF_DECLARE instead of
being a platform driver. Plus, MMSYS provides clocks but also a shared
register space for the mediatek-drm and the mediatek-mdp
driver. So move the MMSYS clocks to a new platform driver and also
create a new MMSYS platform driver in drivers/soc/mediatek that
instantiates the clock driver.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>