clk: gxbb: Document bindings for the GXBB clock controller

Add documentations for the clock controller.
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>

Documentation/devicetree/bindings/clock/amlogic,gxbb-clkc.txt

+* Amlogic GXBB Clock and Reset Unit
+
+The Amlogic GXBB clock controller generates and supplies clock to various
+controllers within the SoC.
+
+Required Properties:
+
+- compatible: should be "amlogic,gxbb-clkc"
+- reg: physical base address of the clock 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 the dt-bindings/clock/gxbb-clkc.h header and can be
+used in device tree sources.
+
+Example: Clock controller node:
+
+	clkc: clock-controller@c883c000 {
+		#clock-cells = <1>;
+		compatible = "amlogic,gxbb-clkc";
+		reg = <0x0 0xc883c000 0x0 0x3db>;
+	};
+
+Example: UART controller node that consumes the clock generated by the clock
+  controller:
+
+	uart_AO: serial@c81004c0 {
+		compatible = "amlogic,meson-uart";
+		reg = <0xc81004c0 0x14>;
+		interrupts = <0 90 1>;
+		clocks = <&clkc CLKID_CLK81>;
+		status = "disabled";
+	};