dt-bindings: add documentation for Exynos3250 clock controller

* Samsung Exynos3250 Clock Controller

The Exynos3250 clock controller generates and supplies clock to various

controllers within the Exynos3250 SoC.

Required Properties:

- compatible: should be one of the following.

  - "samsung,exynos3250-cmu" - controller compatible with Exynos3250 SoC.

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

  region.

- #clock-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

dt-bindings/clock/exynos3250.h header and can be used in device

tree sources.

Example 1: An example of a clock controller node is listed below.

	cmu: clock-controller@10030000 {

		compatible = "samsung,exynos3250-cmu";

		reg = <0x10030000 0x20000>;

		#clock-cells = <1>;

	};

Example 2: UART controller node that consumes the clock generated by the clock

	   controller. Refer to the standard clock bindings for information

	   about 'clocks' and 'clock-names' property.

	serial@13800000 {

		compatible = "samsung,exynos4210-uart";

		reg = <0x13800000 0x100>;

		interrupts = <0 109 0>;

		clocks = <&cmu CLK_UART0>, <&cmu CLK_SCLK_UART0>;

		clock-names = "uart", "clk_uart_baud0";

	};