mfd: Add documentation for atmel-hlcdc DT bindings

The HLCDC IP available on some Atmel SoCs (i.e. at91sam9n12, at91sam9x5
family or sama5d3 family) exposes 2 subdevices:
- a display controller (controlled by a DRM driver)
- a PWM chip

This patch adds documentation for atmel-hlcdc DT bindings.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Anthony Harivel <anthony.harivel@emtrion.de>
Tested-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/atmel-hlcdc.txt

+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>;
+		};
+	};