rtc: digicolor: document device tree binding

Add a device tree binding documentation to the Real Time Clock hardware
block on the Conexant CX92755 SoC.  The CX92755 is from the Digicolor SoCs
series.  Other SoCs in that series may share the same hardware block.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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

+Conexant Digicolor Real Time Clock controller
+
+This binding currently supports the CX92755 SoC.
+
+Required properties:
+- compatible: should be "cnxt,cx92755-rtc"
+- reg: physical base address of the controller and length of memory mapped
+  region.
+- interrupts: rtc alarm interrupt
+
+Example:
+
+	rtc@f0000c30 {
+		compatible = "cnxt,cx92755-rtc";
+		reg = <0xf0000c30 0x18>;
+		interrupts = <25>;
+	};