dt-bindings: rtc: mediatek: add bindings for MediaTek SoC based RTC

Device-Tree bindings for MediaTek SoC based RTC

Required properties:

- compatible	    : Should be

			"mediatek,mt7622-rtc", "mediatek,soc-rtc" : for MT7622 SoC

- reg 		    : Specifies base physical address and size of the registers;

- interrupts	    : Should contain the interrupt for RTC alarm;

- clocks	    : Specifies list of clock specifiers, corresponding to

		      entries in clock-names property;

- clock-names	    : Should contain "rtc" entries

Example:

rtc: rtc@10212800 {

	compatible = "mediatek,mt7622-rtc",

		     "mediatek,soc-rtc";

	reg = <0 0x10212800 0 0x200>;

	interrupts = <GIC_SPI 129 IRQ_TYPE_LEVEL_LOW>;

	clocks = <&topckgen CLK_TOP_RTC>;

	clock-names = "rtc";

};