dt-bindings: Add doc for the Ingenic JZ47xx LCD controller driver

Add documentation for the devicetree bindings of the LCD controller
present in the JZ47xx family of SoCs from Ingenic.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603152331.23160-1-paul@crapouillou.net

Documentation/devicetree/bindings/display/ingenic,lcd.txt

+Ingenic JZ47xx LCD driver
+
+Required properties:
+- compatible: one of:
+  * ingenic,jz4740-lcd
+  * ingenic,jz4725b-lcd
+- reg: LCD registers location and length
+- clocks: LCD pixclock and device clock specifiers.
+	   The device clock is only required on the JZ4740.
+- clock-names: "lcd_pclk" and "lcd"
+- interrupts: Specifies the interrupt line the LCD controller is connected to.
+
+Example:
+
+panel {
+	compatible = "sharp,ls020b1dd01d";
+
+	backlight = <&backlight>;
+	power-supply = <&vcc>;
+
+	port {
+		panel_input: endpoint {
+			remote-endpoint = <&panel_output>;
+		};
+	};
+};
+
+
+lcd: lcd-controller@13050000 {
+	compatible = "ingenic,jz4725b-lcd";
+	reg = <0x13050000 0x1000>;
+
+	interrupt-parent = <&intc>;
+	interrupts = <31>;
+
+	clocks = <&cgu JZ4725B_CLK_LCD>;
+	clock-names = "lcd";
+
+	port {
+		panel_output: endpoint {
+			remote-endpoint = <&panel_input>;
+		};
+	};
+};