dt-bindings: Add mtk-timer bindings

Add binding documentation for the General Purpose Timer driver of
the Mediatek SoCs.
Signed-off-by: NMatthias Brugger <matthias.bgg@gmail.com>
Acked-by: NSören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/mediatek,mtk-timer.txt

+Mediatek MT6577, MT6572 and MT6589 Timers
+---------------------------------------
+
+Required properties:
+- compatible: Should be "mediatek,mt6577-timer"
+- reg: Should contain location and length for timers register.
+- clocks: Clocks driving the timer hardware. This list should include two
+	clocks. The order is system clock and as second clock the RTC clock.
+
+Examples:
+
+	timer@10008000 {
+		compatible = "mediatek,mt6577-timer";
+		reg = <0x10008000 0x80>;
+		interrupts = <GIC_SPI 113 IRQ_TYPE_LEVEL_LOW>;
+		clocks = <&system_clk>, <&rtc_clk>;
+	};