dt-bindings: mrvl,mmp-timer: add clock

The timer needs the timer clock to be enabled, otherwise it stops
ticking.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Olof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/timer/mrvl,mmp-timer.txt

@@ -5,9 +5,13 @@ Required properties:
 - reg : Address and length of the register set of timer controller.
 - interrupts : Should be the interrupt number.
 
+Optional properties:
+- clocks : Should contain a single entry describing the clock input.
+
 Example:
 	timer0: timer@d4014000 {
 		compatible = "mrvl,mmp-timer";
 		reg = <0xd4014000 0x100>;
 		interrupts = <13>;
+		clocks = <&coreclk 2>;
 	};