dt-bindings: document the STM32 RTC bindings

This patch adds documentation of device tree bindings for the STM32 RTC.
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/devicetree/bindings/rtc/st,stm32-rtc.txt

+STM32 Real Time Clock
+
+Required properties:
+- compatible: "st,stm32-rtc".
+- reg: address range of rtc register set.
+- clocks: reference to the clock entry ck_rtc.
+- interrupt-parent: phandle for the interrupt controller.
+- interrupts: rtc alarm interrupt.
+- st,syscfg: phandle for pwrcfg, mandatory to disable/enable backup domain
+  (RTC registers) write protection.
+
+Optional properties (to override default ck_rtc parent clock):
+- assigned-clocks: reference to the ck_rtc clock entry.
+- assigned-clock-parents: phandle of the new parent clock of ck_rtc.
+
+Example:
+
+	rtc: rtc@40002800 {
+		compatible = "st,stm32-rtc";
+		reg = <0x40002800 0x400>;
+		clocks = <&rcc 1 CLK_RTC>;
+		assigned-clocks = <&rcc 1 CLK_RTC>;
+		assigned-clock-parents = <&rcc 1 CLK_LSE>;
+		interrupt-parent = <&exti>;
+		interrupts = <17 1>;
+		st,syscfg = <&pwrcfg>;
+	};