dt-bindings: add documentation of rk3228 clock controller

* Rockchip RK3228 Clock and Reset Unit

The RK3228 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,rk3228-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 changeable, 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/rk3228-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,

 - "ext_i2s" - external I2S clock - optional,

 - "ext_gmac" - external GMAC clock - optional

 - "ext_hsadc" - external HSADC clock - optional

 - "phy_50m_out" - output clock of the pll in the mac phy

Example: Clock controller node:

	cru: cru@20000000 {

		compatible = "rockchip,rk3228-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@10110000 {

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

		reg = <0x10110000 0x100>;

		interrupts = <GIC_SPI 55 IRQ_TYPE_LEVEL_HIGH>;

		reg-shift = <2>;

		reg-io-width = <4>;

		clocks = <&cru SCLK_UART0>;

	};