Documentation: bindings: add dt documentation for dfi controller

This patch adds the documentation for rockchip dfi devfreq-event driver.

Signed-off-by: Lin Huang <[e-mail]>
Acked-by: Chanwoo Choi <[e-mail]>
Signed-off-by: MyungJoo Ham <[e-mail]>

Documentation/devicetree/bindings/devfreq/event/rockchip-dfi.txt

···
+
+* Rockchip rk3399 DFI device
+
+Required properties:
+- compatible: Must be "rockchip,rk3399-dfi".
+- reg: physical base address of each DFI and length of memory mapped region
+- rockchip,pmu: phandle to the syscon managing the "pmu general register files"
+- clocks: phandles for clock specified in "clock-names" property
+- clock-names : the name of clock used by the DFI, must be "pclk_ddr_mon";
+
+Example:
+	dfi: dfi@0xff630000 {
+		compatible = "rockchip,rk3399-dfi";
+		reg = <0x00 0xff630000 0x00 0x4000>;
+		rockchip,pmu = <&pmugrf>;
+		clocks = <&cru PCLK_DDR_MON>;
+		clock-names = "pclk_ddr_mon";
+		status = "disabled";
+	};