dt-bindings: add bindings for rk3399 clock controller

* Rockchip RK3399 Clock and Reset Unit

The RK3399 clock controller generates and supplies clock to various

controllers within the SoC and also implements a reset controller for SoC

peripherals.

Required Properties:

- compatible: PMU for CRU should be "rockchip,rk3399-pmucru"

- compatible: CRU should be "rockchip,rk3399-cru"

- reg: physical base address of the controller and length of memory mapped

  region.

- #clock-cells: should be 1.

- #reset-cells: should be 1.

Each clock is assigned an identifier and client nodes can use this identifier

to specify the clock which they consume. All available clocks are defined as

preprocessor macros in the dt-bindings/clock/rk3399-cru.h headers and can be

used in device tree sources. Similar macros exist for the reset sources in

these files.

External clocks:

There are several clocks that are generated outside the SoC. It is expected

that they are defined using standard clock bindings with following

clock-output-names:

 - "xin24m" - crystal input - required,

 - "xin32k" - rtc clock - optional,

 - "clkin_gmac" - external GMAC clock - optional,

 - "clkin_i2s" - external I2S clock - optional,

 - "pclkin_cif" - external ISP clock - optional,

 - "clk_usbphy0_480m" - output clock of the pll in the usbphy0

 - "clk_usbphy1_480m" - output clock of the pll in the usbphy1

Example: Clock controller node:

	pmucru: pmu-clock-controller@ff750000 {

		compatible = "rockchip,rk3399-pmucru";

		reg = <0x0 0xff750000 0x0 0x1000>;

		#clock-cells = <1>;

		#reset-cells = <1>;

	};

	cru: clock-controller@ff760000 {

		compatible = "rockchip,rk3399-cru";

		reg = <0x0 0xff760000 0x0 0x1000>;

		#clock-cells = <1>;

		#reset-cells = <1>;

	};

Example: UART controller node that consumes the clock generated by the clock

  controller:

	uart0: serial@ff1a0000 {

		compatible = "rockchip,rk3399-uart", "snps,dw-apb-uart";

		reg = <0x0 0xff180000 0x0 0x100>;

		clocks = <&cru SCLK_UART0>, <&cru PCLK_UART0>;

		clock-names = "baudclk", "apb_pclk";

		interrupts = <GIC_SPI 99 IRQ_TYPE_LEVEL_HIGH>;

		reg-shift = <2>;

		reg-io-width = <4>;

	};