ARM: sun4i/sun7i: DT documentation for RTC driver

Add DT documentation for SUNXI RTC driver

Signed-off-by: Carlo Caione <carlo.caione@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

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

@@ -0,0 +1,17 @@
+* sun4i/sun7i Real Time Clock
+
+RTC controller for the Allwinner A10/A20
+
+Required properties:
+- compatible : Should be "allwinner,sun4i-rtc" or "allwinner,sun7i-a20-rtc"
+- reg: physical base address of the controller and length of memory mapped
+  region.
+- interrupts: IRQ line for the RTC.
+
+Example:
+
+rtc: rtc@01c20d00 {
+	compatible = "allwinner,sun4i-rtc";
+	reg = <0x01c20d00 0x20>;
+	interrupts = <24>;
+};