dt-bindings: new binding for Ilitek ILI9341 display panels

This adds a new binding for Ilitek ILI9341 display panels. It includes
a compatible string for one display (more can be added in the future).

The YX240QV29-T panel[1] is found, for example, in an Adafruit breakout
board[2] and in Mindsensors' PiStorms[3].

The vendor prefix "adafruit" is used because the actual vendor is not
known, but Adafruit is the most common source for a product that
contains this panel.

[1]: https://cdn-learn.adafruit.com/assets/assets/000/046/879/original/SPEC-YX240QV29-T_Rev.A__1_.pdf
[2]: https://www.adafruit.com/product/2478
[3]: http://www.mindsensors.com/stem-with-robotics/13-pistorms-v2-base-kit-raspberry-pi-brain-for-lego-robotSigned-off-by: David Lechner <david@lechnology.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525193623.15533-4-david@lechnology.com

Documentation/devicetree/bindings/display/ilitek,ili9341.txt

+Ilitek ILI9341 display panels
+
+This binding is for display panels using an Ilitek ILI9341 controller in SPI
+mode.
+
+Required properties:
+- compatible:	"adafruit,yx240qv29", "ilitek,ili9341"
+- dc-gpios:	D/C pin
+- reset-gpios:	Reset pin
+
+The node for this driver must be a child node of a SPI controller, hence
+all mandatory properties described in ../spi/spi-bus.txt must be specified.
+
+Optional properties:
+- rotation:	panel rotation in degrees counter clockwise (0,90,180,270)
+- backlight:	phandle of the backlight device attached to the panel
+
+Example:
+	display@0{
+		compatible = "adafruit,yx240qv29", "ilitek,ili9341";
+		reg = <0>;
+		spi-max-frequency = <32000000>;
+		dc-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
+		reset-gpios = <&gpio0 8 GPIO_ACTIVE_HIGH>;
+		rotation = <270>;
+		backlight = <&backlight>;
+	};