dt-bindings: Document the ARM System timer bindings

This adds documentation of device tree bindings for the
ARM System timer.
Tested-by: NChanwoo Choi <cw00.choi@samsung.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMaxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/arm/armv7m_systick.txt

+* ARMv7M System Timer
+
+ARMv7-M includes a system timer, known as SysTick. Current driver only
+implements the clocksource feature.
+
+Required properties:
+- compatible	  : Should be "arm,armv7m-systick"
+- reg		  : The address range of the timer
+
+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 SysTick
+
+Examples:
+
+systick: timer@e000e010 {
+	compatible = "arm,armv7m-systick";
+	reg = <0xe000e010 0x10>;
+	clocks = <&clk_systick>;
+};
+
+systick: timer@e000e010 {
+	compatible = "arm,armv7m-systick";
+	reg = <0xe000e010 0x10>;
+	clock-frequency = <90000000>;
+};