dt-bindings: rtc: add bindings for i.MX53 SRTC

Document the binding for i.MX53 SRTC implemented by rtc-mxc_v2
Signed-off-by: NPatrick Bruenn <p.bruenn@beckhoff.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/rtc-mxc_v2.txt

+* i.MX53 Secure Real Time Clock (SRTC)
+
+Required properties:
+- compatible: should be: "fsl,imx53-rtc"
+- reg: physical base address of the controller and length of memory mapped
+  region.
+- clocks: should contain the phandle for the rtc clock
+- interrupts: rtc alarm interrupt
+
+Example:
+
+rtc@53fa4000 {
+	compatible = "fsl,imx53-rtc";
+	reg = <0x53fa4000 0x4000>;
+	interrupts = <24>;
+	clocks = <&clks IMX5_CLK_SRTC_GATE>;
+};