drm/panel: Add DT bindings for Samsung S6D16D0

This adds the device tree bindings for the Samsung S6D16D0
panel. This is a command mode only panel using DSI.

Cc: devicetree@vger.kernel.org
Cc: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023074630.29186-1-linus.walleij@linaro.org

Documentation/devicetree/bindings/display/panel/samsung,s6d16d0.txt

+Samsung S6D16D0 4" 864x480 AMOLED panel
+
+Required properties:
+  - compatible: should be:
+    "samsung,s6d16d0",
+  - reg: the virtual channel number of a DSI peripheral
+  - vdd1-supply: I/O voltage supply
+  - reset-gpios: a GPIO spec for the reset pin (active low)
+
+The device node can contain one 'port' child node with one child
+'endpoint' node, according to the bindings defined in
+media/video-interfaces.txt. This node should describe panel's video bus.
+
+Example:
+&dsi {
+	...
+
+	panel@0 {
+		compatible = "samsung,s6d16d0";
+		reg = <0>;
+		vdd1-supply = <&foo>;
+		reset-gpios = <&foo_gpio 0 GPIO_ACTIVE_LOW>;
+
+		port {
+			panel_in: endpoint {
+				remote-endpoint = <&dsi_out>;
+			};
+		};
+	};
+};