dt-bindings: Add Rocktech jh057n00900 panel bindings

The Rocktec jh057n00900 is a 5.5" MIPI DSI video mode panel with a
720x1440 resolution and a built in backlight.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ec59b22907ac28764ffb3bec33445b6e019945a4.1554114302.git.agx@sigxcpu.org

Documentation/devicetree/bindings/display/panel/rocktech,jh057n00900.txt

+Rocktech jh057n00900 5.5" 720x1440 TFT LCD panel
+
+Required properties:
+- compatible: should be "rocktech,jh057n00900"
+- reg: DSI virtual channel of the peripheral
+- reset-gpios: panel reset gpio
+- backlight: phandle of the backlight device attached to the panel
+
+Example:
+
+	&mipi_dsi {
+		panel@0 {
+			compatible = "rocktech,jh057n00900";
+			reg = <0>;
+			backlight = <&backlight>;
+			reset-gpios = <&gpio3 13 GPIO_ACTIVE_LOW>;
+		};
+	};