drm: add DT bindings documentation for atmel-hlcdc-dc driver

Device-Tree bindings for Atmel's HLCDC (High LCD Controller) DRM driver

The Atmel HLCDC Display Controller is subdevice of the HLCDC MFD device.

See ../mfd/atmel-hlcdc.txt for more details.

Required properties:

 - compatible: value should be "atmel,hlcdc-display-controller"

 - pinctrl-names: the pin control state names. Should contain "default".

 - pinctrl-0: should contain the default pinctrl states.

 - #address-cells: should be set to 1.

 - #size-cells: should be set to 0.

Required children nodes:

 Children nodes are encoding available output ports and their connections

 to external devices using the OF graph reprensentation (see ../graph.txt).

 At least one port node is required.

Example:

	hlcdc: hlcdc@f0030000 {

		compatible = "atmel,sama5d3-hlcdc";

		reg = <0xf0030000 0x2000>;

		interrupts = <36 IRQ_TYPE_LEVEL_HIGH 0>;

		clocks = <&lcdc_clk>, <&lcdck>, <&clk32k>;

		clock-names = "periph_clk","sys_clk", "slow_clk";

		status = "disabled";

		hlcdc-display-controller {

			compatible = "atmel,hlcdc-display-controller";

			pinctrl-names = "default";

			pinctrl-0 = <&pinctrl_lcd_base &pinctrl_lcd_rgb888>;

			#address-cells = <1>;

			#size-cells = <0>;

			port@0 {

				#address-cells = <1>;

				#size-cells = <0>;

				reg = <0>;

				hlcdc_panel_output: endpoint@0 {

					reg = <0>;

					remote-endpoint = <&panel_input>;

				};

			};

		};

		hlcdc_pwm: hlcdc-pwm {

			compatible = "atmel,hlcdc-pwm";

			pinctrl-names = "default";

			pinctrl-0 = <&pinctrl_lcd_pwm>;

			#pwm-cells = <3>;

		};

	};