video: clps711x: Add bindings documentation for CLPS711X framebuffer

* Currus Logic CLPS711X Framebuffer

Required properties:

- compatible: Shall contain "cirrus,clps711x-fb".

- reg       : Physical base address and length of the controller's registers +

              location and size of the framebuffer memory.

- clocks    : phandle + clock specifier pair of the FB reference clock.

- display   : phandle to a display node as described in

              Documentation/devicetree/bindings/video/display-timing.txt.

              Additionally, the display node has to define properties:

  - bits-per-pixel: Bits per pixel.

  - ac-prescale   : LCD AC bias frequency. This frequency is the required

                    AC bias frequency for a given manufacturer's LCD plate.

  - cmap-invert   : Invert the color levels (Optional).

Optional properties:

- lcd-supply: Regulator for LCD supply voltage.

Example:

	fb: fb@800002c0 {

		compatible = "cirrus,ep7312-fb", "cirrus,clps711x-fb";

		reg = <0x800002c0 0xd44>, <0x60000000 0xc000>;

		clocks = <&clks 2>;

		lcd-supply = <&reg5v0>;

		display = <&display>;

	};

	display: display {

		model = "320x240x4";

		native-mode = <&timing0>;

		bits-per-pixel = <4>;

		ac-prescale = <17>;

		display-timings {

			timing0: 320x240 {

				hactive = <320>;

				hback-porch = <0>;

				hfront-porch = <0>;

				hsync-len = <0>;

				vactive = <240>;

				vback-porch = <0>;

				vfront-porch = <0>;

				vsync-len = <0>;

				clock-frequency = <6500000>;

			};

		};

	};