dt-bindings: mediatek: Update mmsys binding to reflect it is a system controller

The mmsys system controller is not only a pure clock controller, so
update the binding documentation to reflect that apart from providing
clocks, it also provides routing and miscellaneous control registers.

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