Documentation: dt: Add binding info for jz4740-rtc driver

This commit adds documentation for the device-tree bindings of the
jz4740-rtc driver, which supports the RTC unit present in the JZ4740 and
JZ4780 SoCs from Ingenic.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Maarten ter Huurne <maarten@treewalker.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt

+JZ4740 and similar SoCs real-time clock driver
+
+Required properties:
+
+- compatible: One of:
+  - "ingenic,jz4740-rtc" - for use with the JZ4740 SoC
+  - "ingenic,jz4780-rtc" - for use with the JZ4780 SoC
+- reg: Address range of rtc register set
+- interrupts: IRQ number for the alarm interrupt
+- clocks: phandle to the "rtc" clock
+- clock-names: must be "rtc"
+
+Optional properties:
+- system-power-controller: To use this component as the
+  system power controller
+- reset-pin-assert-time-ms: Reset pin low-level assertion
+  time after wakeup (default 60ms; range 0-125ms if RTC clock
+  at 32 kHz)
+- min-wakeup-pin-assert-time-ms: Minimum wakeup pin assertion
+  time (default 100ms; range 0-2s if RTC clock at 32 kHz)
+
+Example:
+
+rtc@10003000 {
+	compatible = "ingenic,jz4740-rtc";
+	reg = <0x10003000 0x40>;
+
+	interrupt-parent = <&intc>;
+	interrupts = <32>;
+
+	clocks = <&rtc_clock>;
+	clock-names = "rtc";
+
+	system-power-controller;
+	reset-pin-assert-time-ms = <60>;
+	min-wakeup-pin-assert-time-ms = <100>;
+};