dt-bindings: add documentation for s3c2412 clock controller

Describe the clock controller of the s3c2412.

Signed-off-by: Heiko Stuebner <[e-mail]>
Signed-off-by: Kukjin Kim <[e-mail]>

Documentation/devicetree/bindings/clock/samsung,s3c2412-clock.txt

···
+* Samsung S3C2412 Clock Controller
+
+The S3C2412 clock controller generates and supplies clock to various controllers
+within the SoC. The clock binding described here is applicable to the s3c2412
+and s3c2413 SoCs in the s3c24x family.
+
+Required Properties:
+
+- compatible: should be "samsung,s3c2412-clock"
+- 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. Some of the clocks are available only
+on a particular SoC.
+
+All available clocks are defined as preprocessor macros in
+dt-bindings/clock/s3c2412.h header and can be used in device
+tree sources.
+
+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:
+ - "xti" - crystal input - required,
+ - "ext" - external clock source - optional,
+
+Example: Clock controller node:
+
+	clocks: clock-controller@4c000000 {
+		compatible = "samsung,s3c2412-clock";
+		reg = <0x4c000000 0x20>;
+		#clock-cells = <1>;
+	};
+
+Example: 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" properties):
+
+	serial@50004000 {
+		compatible = "samsung,s3c2412-uart";
+		reg = <0x50004000 0x4000>;
+		interrupts = <1 23 3 4>, <1 23 4 4>;
+		clock-names = "uart", "clk_uart_baud2", "clk_uart_baud3";
+		clocks = <&clocks PCLK_UART0>, <&clocks PCLK_UART0>,
+			 <&clocks SCLK_UART>;
+		status = "disabled";
+	};