drm/panel: Device tree bindings for ARM Versatile panels

This adds a pretty simple set of device tree bindings for
ARM Versatile panels appearing as child nodes of a system
controller.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205192013.5349-1-linus.walleij@linaro.org

Documentation/devicetree/bindings/display/panel/arm,versatile-tft-panel.txt

+ARM Versatile TFT Panels
+
+These panels are connected to the daughterboards found on the
+ARM Versatile reference designs.
+
+This device node must appear as a child to a "syscon"-compatible
+node.
+
+Required properties:
+- compatible: should be "arm,versatile-tft-panel"
+
+Required subnodes:
+- port: see display/panel/panel-common.txt, graph.txt
+
+
+Example:
+
+sysreg@0 {
+	compatible = "arm,versatile-sysreg", "syscon", "simple-mfd";
+	reg = <0x00000 0x1000>;
+
+	panel: display@0 {
+		compatible = "arm,versatile-tft-panel";
+
+		port {
+			panel_in: endpoint {
+				remote-endpoint = <&foo>;
+			};
+		};
+	};
+};