dt-bindings: rtc: Add bindings for Cadence RTC

This patch adds a DT binding documentation for
Cadence RTC controller.
Signed-off-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/cdns,rtc.txt

+Cadence Real Time Clock
+
+The Cadence RTC controller with date, time and alarm capabilities.
+The alarm may wake the system from low-power state.
+
+Required properties:
+- compatible: Should be "cdns,rtc-r109v3"
+- reg: Specifies base physical address and size of the register area.
+- interrupts: A single interrupt specifier.
+- clocks: Must contain two entries:
+	- pclk: APB registers clock
+	- ref_clk: reference 1Hz or 100Hz clock, depending on IP configuration
+	See ../clocks/clock-bindings.txt for details.
+
+Example:
+        rtc0: rtc@fd080000 {
+        	compatible = "cdns,rtc-r109v3";
+        	reg = <0xfd080000 0x1000>;
+
+        	clock-names = "pclk", "ref_clk";
+        	clocks = <&sysclock>, <&refclock>;
+
+        	interrupt-parent = <&gic>;
+        	interrupts = <0 6 IRQ_TYPE_LEVEL_HIGH>;
+        };