rtc: rtc-mv: Add the device tree binding documentation

The support was already written, but the binding documentation was
lacking.
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

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

+* Mvebu Real Time Clock
+
+RTC controller for the Kirkwood, the Dove, the Armada 370 and the
+Armada XP SoCs
+
+Required properties:
+- compatible : Should be "marvell,orion-rtc"
+- reg: physical base address of the controller and length of memory mapped
+  region.
+- interrupts: IRQ line for the RTC.
+
+Example:
+
+rtc@10300 {
+        compatible = "marvell,orion-rtc";
+        reg = <0xd0010300 0x20>;
+        interrupts = <50>;
+};