Documentation: drm: Add DT bindings for ARM HDLCD

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>

Documentation/devicetree/bindings/display/arm,hdlcd.txt

+ARM HDLCD
+
+This is a display controller found on several development platforms produced
+by ARM Ltd and in more modern of its' Fast Models. The HDLCD is an RGB
+streamer that reads the data from a framebuffer and sends it to a single
+digital encoder (DVI or HDMI).
+
+Required properties:
+  - compatible: "arm,hdlcd"
+  - reg: Physical base address and length of the controller's registers.
+  - interrupts: One interrupt used by the display controller to notify the
+    interrupt controller when any of the interrupt sources programmed in
+    the interrupt mask register have activated.
+  - clocks: A list of phandle + clock-specifier pairs, one for each
+    entry in 'clock-names'.
+  - clock-names: A list of clock names. For HDLCD it should contain:
+      - "pxlclk" for the clock feeding the output PLL of the controller.
+
+Required sub-nodes:
+  - port: The HDLCD connection to an encoder chip. The connection is modeled
+    using the OF graph bindings specified in
+    Documentation/devicetree/bindings/graph.txt.
+
+Optional properties:
+  - memory-region: phandle to a node describing memory (see
+    Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt) to be
+    used for the framebuffer; if not present, the framebuffer may be located
+    anywhere in memory.
+
+
+Example:
+
+/ {
+	...
+
+	hdlcd@2b000000 {
+		compatible = "arm,hdlcd";
+		reg = <0 0x2b000000 0 0x1000>;
+		interrupts = <GIC_SPI 85 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&oscclk5>;
+		clock-names = "pxlclk";
+		port {
+			hdlcd_output: endpoint@0 {
+				remote-endpoint = <&hdmi_enc_input>;
+			};
+		};
+	};
+
+	/* HDMI encoder on I2C bus */
+	i2c@7ffa0000 {
+		....
+		hdmi-transmitter@70 {
+			compatible = ".....";
+			reg = <0x70>;
+			port@0 {
+				hdmi_enc_input: endpoint {
+					remote-endpoint = <&hdlcd_output>;
+				};
+
+				hdmi_enc_output: endpoint {
+					remote-endpoint = <&hdmi_1_port>;
+				};
+			};
+		};
+
+	};
+
+	hdmi1: connector@1 {
+		compatible = "hdmi-connector";
+		type = "a";
+		port {
+			hdmi_1_port: endpoint {
+				remote-endpoint = <&hdmi_enc_output>;
+			};
+		};
+	};
+
+	...
+};