drm/panel: Add DT bindings for Novatek NT35510-based panels

This adds device tree bindings for the Novatek NT35510-based
family of panels. Since several such panels are in existence
we define bindings common for all, and define the compatible
string for one certain panel (Hydis HVA40WV1).

As other panels are discovered and investigated, we can add
more compatibles to the binding using oneOf constructions.

Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200223121841.26836-2-linus.walleij@linaro.org

Documentation/devicetree/bindings/display/panel/novatek,nt35510.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/novatek,nt35510.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Novatek NT35510-based display panels
+
+maintainers:
+  - Linus Walleij <linus.walleij@linaro.org>
+
+allOf:
+  - $ref: panel-common.yaml#
+
+properties:
+  compatible:
+    items:
+      - const: hydis,hva40wv1
+      - const: novatek,nt35510
+    description: This indicates the panel manufacturer of the panel
+      that is in turn using the NT35510 panel driver. The compatible
+      string determines how the NT35510 panel driver shall be configured
+      to work with the indicated panel. The novatek,nt35510 compatible shall
+      always be provided as a fallback.
+  reg: true
+  reset-gpios: true
+  vdd-supply:
+     description: regulator that supplies the vdd voltage
+  vddi-supply:
+     description: regulator that supplies the vddi voltage
+  backlight: true
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    dsi@a0351000 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        panel {
+            compatible = "hydis,hva40wv1", "novatek,nt35510";
+            reg = <0>;
+            vdd-supply = <&ab8500_ldo_aux4_reg>;
+            vddi-supply = <&ab8500_ldo_aux6_reg>;
+            reset-gpios = <&gpio4 11 GPIO_ACTIVE_LOW>;
+            backlight = <&gpio_bl>;
+        };
+    };
+
+...