RTC: add DT bindings to pxa-rtc

This patch adds generic device tree bindings to the PXA RTC driver.
Documentation for bindings were also added.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>

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

@@ -0,0 +1,14 @@
+* PXA RTC
+
+PXA specific RTC driver.
+
+Required properties:
+- compatible : Should be "marvell,pxa-rtc"
+
+Examples:
+
+rtc@40900000 {
+	compatible = "marvell,pxa-rtc";
+	reg = <0x40900000 0x3c>;
+	interrupts = <30 31>;
+};