dt-bindings: Add Truly NT35597 panel driver bindings

Add the device tree bindings for Truly NT35597 panel driver. This panel
driver supports both single DSI and dual DSI.

However, this patch series supports only dual DSI.

Changes in v10:
  - Remove active high in the documentation for mode gpio
  - Fix commmit text detail about the board and also
    fix a typo

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1538787139-28122-2-git-send-email-abhinavk@codeaurora.org

Documentation/devicetree/bindings/display/truly,nt35597.txt

+Truly model NT35597 DSI display driver
+
+The Truly NT35597 is a generic display driver, currently only configured
+for use in the 2K display on the Qualcomm SDM845 MTP board.
+
+Required properties:
+- compatible: should be "truly,nt35597-2K-display"
+- vdda-supply: phandle of the regulator that provides the supply voltage
+  Power IC supply
+- vdispp-supply: phandle of the regulator that provides the supply voltage
+  for positive LCD bias
+- vdispn-supply: phandle of the regulator that provides the supply voltage
+  for negative LCD bias
+- reset-gpios: phandle of gpio for reset line
+  This should be 8mA, gpio can be configured using mux, pinctrl, pinctrl-names
+  (active low)
+- mode-gpios: phandle of the gpio for choosing the mode of the display
+  for single DSI or Dual DSI
+  This should be low for dual DSI and high for single DSI mode
+- ports: This device has two video ports driven by two DSIs. Their connections
+  are modeled using the OF graph bindings specified in
+  Documentation/devicetree/bindings/graph.txt.
+  - port@0: DSI input port driven by master DSI
+  - port@1: DSI input port driven by secondary DSI
+
+Example:
+
+	dsi@ae94000 {
+		panel@0 {
+			compatible = "truly,nt35597-2K-display";
+			reg = <0>;
+			vdda-supply = <&pm8998_l14>;
+			vdispp-supply = <&lab_regulator>;
+			vdispn-supply = <&ibb_regulator>;
+			pinctrl-names = "default", "suspend";
+			pinctrl-0 = <&dpu_dsi_active>;
+			pinctrl-1 = <&dpu_dsi_suspend>;
+
+			reset-gpios = <&tlmm 6 GPIO_ACTIVE_LOW>;
+			mode-gpios = <&tlmm 52 GPIO_ACTIVE_HIGH>;
+			ports {
+				#address-cells = <1>;
+				#size-cells = <0>;
+				port@0 {
+					reg = <0>;
+					panel0_in: endpoint {
+						remote-endpoint = <&dsi0_out>;
+					};
+				};
+
+				port@1 {
+					reg = <1>;
+					panel1_in: endpoint {
+						remote-endpoint = <&dsi1_out>;
+					};
+				};
+			};
+		};
+	};