RTC Reboot Mode Configuration

Required Properties:
- compatible: must be "reboot-mode-rtc".
- rtc: reference to the rtc device used.
- reg: start register and the number of bytes used. Maximum 4 bytes supported.
- modes: list of properties that define the modes and associated unique ids.

Optional Properties:
- u-boot,env-variable: used to save the reboot mode (default: reboot-mode).
- big-endian: if the magic value is stored in big-endian. (default: false).

Example:
	reboot-mode-rtc {
		compatible = "reboot-mode-rtc";
		rtc = <&rtc_0>;
		reg = <0x14 4>;
		u-boot,env-variable = "bootstatus";
		big-endian;
		modes-test = <0x21969147>;
		modes-download = <0x51939147>;
	};