dt-bindings: rtc: add bindings for MT2712 RTC

Document the binding for MT2712 RTC implemented by rtc-mt2712.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ran Bi <ran.bi@mediatek.com>
Link: https://lore.kernel.org/r/20200226051303.22560-2-ran.bi@mediatek.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

Documentation/devicetree/bindings/rtc/rtc-mt2712.txt

+Device-Tree bindings for MediaTek SoC based RTC
+
+Required properties:
+- compatible	    : Should be "mediatek,mt2712-rtc" : for MT2712 SoC
+- reg 		    : Specifies base physical address and size of the registers;
+- interrupts	    : Should contain the interrupt for RTC alarm;
+
+Example:
+
+rtc: rtc@10011000 {
+	compatible = "mediatek,mt2712-rtc";
+	reg = <0 0x10011000 0 0x1000>;
+	interrupts = <GIC_SPI 239 IRQ_TYPE_LEVEL_LOW>;
+};