Documentation: bindings: add dt documentation for dfi controller

* 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";

	};