mfd: Add documentation for atmel-hlcdc DT bindings

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

Required properties:

 - compatible: value should be one of the following:

   "atmel,sama5d3-hlcdc"

 - reg: base address and size of the HLCDC device registers.

 - clock-names: the name of the 3 clocks requested by the HLCDC device.

   Should contain "periph_clk", "sys_clk" and "slow_clk".

 - clocks: should contain the 3 clocks requested by the HLCDC device.

 - interrupts: should contain the description of the HLCDC interrupt line

The HLCDC IP exposes two subdevices:

 - a PWM chip: see ../pwm/atmel-hlcdc-pwm.txt

 - a Display Controller: see ../drm/atmel-hlcdc-dc.txt

Example:

	hlcdc: hlcdc@f0030000 {

		compatible = "atmel,sama5d3-hlcdc";

		reg = <0xf0030000 0x2000>;

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

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

		interrupts = <36 IRQ_TYPE_LEVEL_HIGH 0>;

		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>;

		};

	};