drm: Add DT bindings documentation for ARC PGU display controller

This add DT bindings documentation for ARC PGU display controller.
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
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>
Cc: devicetree@vger.kernel.org
Cc: linux-snps-arc@lists.infradead.org
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/display/snps,arcpgu.txt

+ARC PGU
+
+This is a display controller found on several development boards produced
+by Synopsys. The ARC PGU is an RGB streamer that reads the data from a
+framebuffer and sends it to a single digital encoder (usually HDMI).
+
+Required properties:
+  - compatible: "snps,arcpgu"
+  - reg: Physical base address and length of the controller's registers.
+  - clocks: A list of phandle + clock-specifier pairs, one for each
+    entry in 'clock-names'.
+  - clock-names: A list of clock names. For ARC PGU it should contain:
+      - "pxlclk" for the clock feeding the output PLL of the controller.
+
+Required sub-nodes:
+  - port: The PGU connection to an encoder chip.
+
+Example:
+
+/ {
+	...
+
+	pgu@XXXXXXXX {
+		compatible = "snps,arcpgu";
+		reg = <0xXXXXXXXX 0x400>;
+		clocks = <&clock_node>;
+		clock-names = "pxlclk";
+
+		port {
+			pgu_output: endpoint {
+				remote-endpoint = <&hdmi_enc_input>;
+			};
+		};
+	};
+};