STM32 Real Time Clock

Required properties:
- compatible: can be either "st,stm32-rtc" or "st,stm32h7-rtc", depending on
  the device is compatible with stm32(f4/f7) or stm32h7.
- reg: address range of rtc register set.
- clocks: can use up to two clocks, depending on part used:
  - "rtc_ck": RTC clock source.
    It is required on stm32(f4/f7) and stm32h7.
  - "pclk": RTC APB interface clock.
    It is not present on stm32(f4/f7).
    It is required on stm32h7.
- clock-names: must be "rtc_ck" and "pclk".
    It is required only on stm32h7.
- 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 rtc_ck parent clock):
- assigned-clocks: reference to the rtc_ck clock entry.
- assigned-clock-parents: phandle of the new parent clock of rtc_ck.

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>;
	};

	rtc: rtc@58004000 {
		compatible = "st,stm32h7-rtc";
		reg = <0x58004000 0x400>;
		clocks = <&rcc RTCAPB_CK>, <&rcc RTC_CK>;
		clock-names = "pclk", "rtc_ck";
		assigned-clocks = <&rcc RTC_CK>;
		assigned-clock-parents = <&rcc LSE_CK>;
		interrupt-parent = <&exti>;
		interrupts = <17 1>;
		interrupt-names = "alarm";
		st,syscfg = <&pwrcfg>;
	};
