at91: video: DT binding for HLCDC driver

DT binding documentation for atmel HLCDC driver.

Signed-off-by: Songjun Wu <songjun.wu@microchip.com>

doc/device-tree-bindings/video/atmel-hlcdc.txt

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+Atmel HLCDC Framebuffer
+-----------------------------------------------------
+Required properties:
+- compatible :
+	"atmel,sama5d2-hlcdc", "atmel,at91sam9x5-hlcdc".
+- reg: physical base address of the controller and length of memory mapped
+       region.
+- clocks: phandles to input clocks.
+- atmel,vl-bpix: Bits per pixel.
+- atmel,output-mode: LCD Controller Output Mode,
+  The unit is bits per pixel, there are four values,
+  <12>, <16>, <18>, <24>, the default value is <24>.
+- atmel,guard-time: lcd guard time (Delay in frame periods).
+- display-timings: please refer the displaymode.txt.
+
+Example:
+hlcdc: hlcdc@f0000000 {
+	u-boot,dm-pre-reloc;
+	compatible = "atmel,sama5d2-hlcdc";
+	reg = <0xf0000000 0x2000>;
+	clocks = <&lcdc_clk>;
+	atmel,vl-bpix = <4>;
+	atmel,output-mode = <24>;
+	atmel,guard-time = <1>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_lcd_base &pinctrl_lcd_pwm &pinctrl_lcd_rgb666>;
+	status = "okay";
+
+	display-timings {
+		480x272 {
+			clock-frequency = <9000000>;
+			hactive = <480>;
+			vactive = <272>;
+			hsync-len = <41>;
+			hfront-porch = <2>;
+			hback-porch = <2>;
+			vfront-porch = <2>;
+			vback-porch = <2>;
+			vsync-len = <11>;
+		};
+	};
+};