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 <[e-mail]>
Reviewed-by: Matthias Brugger <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Signed-off-by: Matthias Brugger <[e-mail]>

Documentation/devicetree/bindings/arm/mediatek/mediatek,mmsys.txt

···
 Mediatek mmsys controller
 ============================
 
-The Mediatek mmsys controller provides various clocks to the system.
+The Mediatek mmsys system controller provides clock control, routing control,
+and miscellaneous control in mmsys partition.
 
 Required Properties:
 
···
 	- "mediatek,mt8183-mmsys", "syscon"
 - #clock-cells: Must be 1
 
-The mmsys controller uses the common clk binding from
+For the clock control, the mmsys controller uses the common clk binding from
 Documentation/devicetree/bindings/clock/clock-bindings.txt
 The available clocks are defined in dt-bindings/clock/mt*-clk.h.
 
 Example:
 
-mmsys: clock-controller@14000000 {
+mmsys: syscon@14000000 {
 	compatible = "mediatek,mt8173-mmsys", "syscon";
 	reg = <0 0x14000000 0 0x1000>;
 	#clock-cells = <1>;