dt-bindings: rtc: Add sirf,prima2-sysrtc bindings

Add device tree bindings for the SiRFSoC Real Time Clock.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/rtc/sirf,prima2-sysrtc.txt

+SiRFSoC Real Time Clock
+
+Required properties:
+- compatible: must be "sirf,prima2-sysrtc"
+- reg: address range of rtc register set.
+- interrupts: rtc alarm interrupts.
+
+Example:
+	rtc@2000 {
+		compatible = "sirf,prima2-sysrtc";
+		reg = <0x2000 0x1000>;
+		interrupts = <52 53 54>;
+	};