dt-bindings: add documentation for rk3188 clock and reset unit

* Rockchip RK3188/RK3066 Clock and Reset Unit

The RK3188/RK3066 clock controller generates and supplies clock to various

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

peripherals.

Required Properties:

- compatible: should be "rockchip,rk3188-cru", "rockchip,rk3188a-cru" or

			"rockchip,rk3066a-cru"

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

  region.

- #clock-cells: should be 1.

- #reset-cells: should be 1.

Optional Properties:

- rockchip,grf: phandle to the syscon managing the "general register files"

  If missing pll rates are not changable, due to the missing pll lock status.

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/rk3188-cru.h and

dt-bindings/clock/rk3066-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,

 - "xin27m" - 27mhz crystal input on rk3066 - optional,

 - "ext_hsadc" - external HSADC clock - optional,

 - "ext_cif0" - external camera clock - optional,

 - "ext_rmii" - external RMII clock - optional,

 - "ext_jtag" - externalJTAG clock - optional

Example: Clock controller node:

	cru: cru@20000000 {

		compatible = "rockchip,rk3188-cru";

		reg = <0x20000000 0x1000>;

		rockchip,grf = <&grf>;

		#clock-cells = <1>;

		#reset-cells = <1>;

	};

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

  controller:

	uart0: serial@10124000 {

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

		reg = <0x10124000 0x400>;

		interrupts = <GIC_SPI 34 IRQ_TYPE_LEVEL_HIGH>;

		reg-shift = <2>;

		reg-io-width = <1>;

		clocks = <&cru SCLK_UART0>;

	};

== Gate clocks ==

These bindings are deprecated!

Please use the soc specific CRU bindings instead.

The gate registers form a continuos block which makes the dt node

structure a matter of taste, as either all gates can be put into

one gate clock spanning all registers or they can be divided into