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>;
	};