dt-bindings: panel: Add the Ilitek ILI9881c panel documentation

The LHR050H41 from BananaPi is a 1280x700 4-lanes DSI panel based on the
ILI9881c from Ilitek.
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a348cdd07d3287e8203ee8d840ea279fe10a6204.1527587352.git-series.maxime.ripard@bootlin.com

Documentation/devicetree/bindings/display/panel/ilitek,ili9881c.txt

+Ilitek ILI9881c based MIPI-DSI panels
+
+Required properties:
+  - compatible: must be "ilitek,ili9881c" and one of:
+    * "bananapi,lhr050h41"
+  - reg: DSI virtual channel used by that screen
+  - power-supply: phandle to the power regulator
+  - reset-gpios: a GPIO phandle for the reset pin
+
+Optional properties:
+  - backlight: phandle to the backlight used
+
+Example:
+panel@0 {
+	compatible = "bananapi,lhr050h41", "ilitek,ili9881c";
+	reg = <0>;
+	power-supply = <&reg_display>;
+	reset-gpios = <&r_pio 0 5 GPIO_ACTIVE_LOW>; /* PL05 */
+	backlight = <&pwm_bl>;
+};