of: Add simple panel device tree binding

This binding specifies a set of common properties for display panels. It
can be used as a basis by bindings for specific panels.

Bindings for three specific panels are provided to show how the simple
panel binding can be used.
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/panel/auo,b101aw03.txt

+AU Optronics Corporation 10.1" WSVGA TFT LCD panel
+
+Required properties:
+- compatible: should be "auo,b101aw03"
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.

Documentation/devicetree/bindings/panel/chunghwa,claa101wb03.txt

+Chunghwa Picture Tubes Ltd. 10.1" WXGA TFT LCD panel
+
+Required properties:
+- compatible: should be "chunghwa,claa101wb03"
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.

Documentation/devicetree/bindings/panel/panasonic,vvx10f004b00.txt

+Panasonic Corporation 10.1" WUXGA TFT LCD panel
+
+Required properties:
+- compatible: should be "panasonic,vvx10f004b00"
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.

Documentation/devicetree/bindings/panel/simple-panel.txt

+Simple display panel
+
+Required properties:
+- power-supply: regulator to provide the supply voltage
+
+Optional properties:
+- ddc-i2c-bus: phandle of an I2C controller used for DDC EDID probing
+- enable-gpios: GPIO pin to enable or disable the panel
+- backlight: phandle of the backlight device attached to the panel
+
+Example:
+
+	panel: panel {
+		compatible = "cptt,claa101wb01";
+		ddc-i2c-bus = <&panelddc>;
+
+		power-supply = <&vdd_pnl_reg>;
+		enable-gpios = <&gpio 90 0>;
+
+		backlight = <&backlight>;
+	};