dt-bindings: document the MPS2 timer bindings

This adds documentation of device tree bindings for the
timers found on ARM MPS2 platform.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NVladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/arm,mps2-timer.txt

+ARM MPS2 timer
+
+The MPS2 platform has simple general-purpose 32 bits timers.
+
+Required properties:
+- compatible	: Should be "arm,mps2-timer"
+- reg		: Address and length of the register set
+- interrupts	: Reference to the timer interrupt
+
+Required clocking property, have to be one of:
+- clocks	  : The input clock of the timer
+- clock-frequency : The rate in HZ in input of the ARM MPS2 timer
+
+Examples:
+
+timer1: mps2-timer@40000000 {
+	compatible = "arm,mps2-timer";
+	reg = <0x40000000 0x1000>;
+	interrupts = <8>;
+	clocks = <&sysclk>;
+};
+
+timer2: mps2-timer@40001000 {
+	compatible = "arm,mps2-timer";
+	reg = <0x40001000 0x1000>;
+	interrupts = <9>;
+	clock-frequency = <25000000>;
+};