ARM: i.MX: Add GPT devicetree Documentation

The GPT binding is already used on i.MX6 and i.MX25, but not yet
documented. Add a binding document for it.
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

Documentation/devicetree/bindings/timer/fsl,imxgpt.txt

+Freescale i.MX General Purpose Timer (GPT)
+
+Required properties:
+
+- compatible : should be "fsl,<soc>-gpt"
+- reg : Specifies base physical address and size of the registers.
+- interrupts : A list of 4 interrupts; one per timer channel.
+- clocks : The clocks provided by the SoC to drive the timer.
+
+Example:
+
+gpt1: timer@10003000 {
+	compatible = "fsl,imx27-gpt", "fsl,imx1-gpt";
+	reg = <0x10003000 0x1000>;
+	interrupts = <26>;
+	clocks = <&clks 46>, <&clks 61>;
+	clock-names = "ipg", "per";
+};