dt-bindings: display: panel: Add support for Raydium RM68200 DSI panel

The Raydium Semiconductor Corporation RM68200 is a 5.5" 720x1280
TFT LCD panel connected using a MIPI-DSI video interface.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302153222.4377-2-philippe.cornu@st.com

Documentation/devicetree/bindings/display/panel/raydium,rm68200.txt

+Raydium Semiconductor Corporation RM68200 5.5" 720p MIPI-DSI TFT LCD panel
+
+The Raydium Semiconductor Corporation RM68200 is a 5.5" 720x1280 TFT LCD
+panel connected using a MIPI-DSI video interface.
+
+Required properties:
+  - compatible: "raydium,rm68200"
+  - reg: the virtual channel number of a DSI peripheral
+
+Optional properties:
+  - reset-gpios: a GPIO spec for the reset pin (active low).
+  - power-supply: phandle of the regulator that provides the supply voltage.
+  - backlight: phandle of the backlight device attached to the panel.
+
+Example:
+&dsi {
+	...
+	panel@0 {
+		compatible = "raydium,rm68200";
+		reg = <0>;
+		reset-gpios = <&gpiof 15 GPIO_ACTIVE_LOW>;
+		power-supply = <&v1v8>;
+		backlight = <&pwm_backlight>;
+	};
+};