Spreadtrum SC27xx Real Time Clock

Required properties:
- compatible: should be "sprd,sc2731-rtc".
- reg: address offset of rtc register.
- interrupt-parent: phandle for the interrupt controller.
- interrupts: rtc alarm interrupt.

Example:

	sc2731_pmic: pmic@0 {
		compatible = "sprd,sc2731";
		reg = <0>;
		spi-max-frequency = <26000000>;
		interrupts = <GIC_SPI 31 IRQ_TYPE_LEVEL_HIGH>;
		interrupt-controller;
		#interrupt-cells = <2>;
		#address-cells = <1>;
		#size-cells = <0>;

		rtc@280 {
			compatible = "sprd,sc2731-rtc";
			reg = <0x280>;
			interrupt-parent = <&sc2731_pmic>;
			interrupts = <2 IRQ_TYPE_LEVEL_HIGH>;
		};
	};