dt-bindings: display: convert panel-dpi to DT schema

With panel-timing converted, now convert the single
remaining .txt user in panel/ of panel-timing to DT schema.

v2:
  - Drop Thierry as maintainer, as this is not a general panel binding
    and I have no acks.
  - Drop requirement for a panel- specific binding - "panel-dpi" is enough
  - Updated example

v3:
  - added yaml document terminator "..."
  - always require a specific binding - panel-dpi (based on feedback from Rob)
  - use "power-supply" for the supply property, and made it mandatory
    "power-supply" is the standard property for panels

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200216181513.28109-4-sam@ravnborg.org

Documentation/devicetree/bindings/display/panel/panel-dpi.yaml

@@ -0,0 +1,72 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/panel-dpi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic MIPI DPI Panel
+
+maintainers:
+  - Sam Ravnborg <sam@ravnborg.org>
+
+allOf:
+  - $ref: panel-common.yaml#
+
+properties:
+  compatible:
+    description:
+      Shall contain a panel specific compatible and "panel-dpi"
+      in that order.
+    items:
+      - {}
+      - const: panel-dpi
+
+  backlight: true
+  enable-gpios: true
+  height-mm: true
+  label: true
+  panel-timing: true
+  port: true
+  power-supply: true
+  reset-gpios: true
+  width-mm: true
+
+required:
+  - panel-timing
+  - power-supply
+
+additionalProperties: false
+
+examples:
+  - |
+    panel@0 {
+        compatible = "osddisplays,osd057T0559-34ts", "panel-dpi";
+        label = "osddisplay";
+        power-supply = <&vcc_supply>;
+
+        backlight = <&backlight>;
+
+        port {
+            lcd_in: endpoint {
+                remote-endpoint = <&dpi_out>;
+            };
+        };
+        panel-timing {
+            clock-frequency = <9200000>;
+            hactive = <800>;
+            vactive = <480>;
+            hfront-porch = <8>;
+            hback-porch = <4>;
+            hsync-len = <41>;
+            vback-porch = <2>;
+            vfront-porch = <4>;
+            vsync-len = <10>;
+
+            hsync-active = <0>;
+            vsync-active = <0>;
+            de-active = <1>;
+            pixelclk-active = <1>;
+        };
+    };
+
+...