dt-bindings: add binding for Sitronix ST7586 display panels

This adds a new binding for Sitronix ST7586 display panels.

Using lego as the vendor prefix in the compatible string because the display
panel I am working with is an integral part of the LEGO MINDSTORMS EV3.
Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1502127581-10517-3-git-send-email-david@lechnology.com

Documentation/devicetree/bindings/display/sitronix,st7586.txt

+Sitronix ST7586 display panel
+
+Required properties:
+- compatible:	"lego,ev3-lcd".
+- a0-gpios:	The A0 signal (since this binding is for serial mode, this is
+                the pin labeled D1 on the controller, not the pin labeled A0)
+- 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)
+
+Example:
+	display@0{
+		compatible = "lego,ev3-lcd";
+		reg = <0>;
+		spi-max-frequency = <10000000>;
+		a0-gpios = <&gpio 43 GPIO_ACTIVE_HIGH>;
+		reset-gpios = <&gpio 80 GPIO_ACTIVE_HIGH>;
+	};