dt-bindings: one binding file for all simple panels

There is an increasing number of new simple panels.
Common for many of these simple panels are that they have one
mandatory power-supply and some of them have backlight and / or
an enable gpio.

The binding file to describe these panels adds overhead
that really do not add value.
The binding are known and there is nothing gained from a
dedicated binding file nor for any dedicated example.

The following patch introduces a single panel-simple.yaml
and converts two ampire bindings over to the new file.

The conversion - if applied will have following effects:

- The maintainer for the individual file will change
    There is no need for many different maintainers for a simple binding.
    We have the same situation with the panel-simple driver in the kernel.

- The license will change to (GPL-2.0-only OR BSD-2-Clause)
    There is usually only a single line copied from the original
    file, a line that is often copied from a datasheet.
    This license change should be acceptable considered what little
    is copied.
    If the license change is not OK we can use a dedicated binding
    file in these cases.

This is a follow-up on Rob's big patch converting a lot of panel bindings
to individual files:

"dt-bindings: display: Convert a bunch of panels to DT schema"
https://patchwork.ozlabs.org/patch/1197683/

The objectives with one file for the relevant simple panels are:
- Make it simpler to add bindings for simple panels
- Keep the number of bindings file lower and thus easier to find a
  relevant file to copy from when adding new panels.
- Keep the binding documentation for simple panels more consistent
- Make it simpler to add support for new panels

v3:
  - Whitespace fix (checkpatch)
  - Indent compatible (Rob h)
  - Indent comments to give compatible more visibility (Rob h)

v2:
  - spelling fixes (imirkin via irc, Rob)
  - updated description (Rob)
  - list properires in alphabetical order
  - added power-supply to example (Rob)
  - updated title
  - reworded changelog a little
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: devicetree@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200102101712.5085-2-sam@ravnborg.org

Documentation/devicetree/bindings/display/panel/ampire,am-480272h3tmqw-t01h.yaml

-# SPDX-License-Identifier: GPL-2.0
-%YAML 1.2
----
-$id: http://devicetree.org/schemas/display/panel/ampire,am-480272h3tmqw-t01h.yaml#
-$schema: http://devicetree.org/meta-schemas/core.yaml#
-
-title: Ampire AM-480272H3TMQW-T01H 4.3" WQVGA TFT LCD panel
-
-maintainers:
-  - Yannick Fertre <yannick.fertre@st.com>
-  - Thierry Reding <treding@nvidia.com>
-
-allOf:
-  - $ref: panel-common.yaml#
-
-properties:
-  compatible:
-    const: ampire,am-480272h3tmqw-t01h
-
-  power-supply: true
-  enable-gpios: true
-  backlight: true
-  port: true
-
-required:
-  - compatible
-
-additionalProperties: false
-
-examples:
-  - |
-    panel_rgb: panel {
-      compatible = "ampire,am-480272h3tmqw-t01h";
-      enable-gpios = <&gpioa 8 1>;
-      port {
-        panel_in_rgb: endpoint {
-          remote-endpoint = <&controller_out_rgb>;
-        };
-      };
-    };
-
-...

Documentation/devicetree/bindings/display/panel/ampire,am800480r3tmqwa1h.txt

-Ampire AM-800480R3TMQW-A1H 7.0" WVGA TFT LCD panel
-
-Required properties:
-- compatible: should be "ampire,am800480r3tmqwa1h"
-
-This binding is compatible with the simple-panel binding, which is specified
-in simple-panel.txt in this directory.

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

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/panel-simple.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Simple panels with one power supply
+
+maintainers:
+  - Thierry Reding <thierry.reding@gmail.com>
+  - Sam Ravnborg <sam@ravnborg.org>
+
+description: |
+  This binding file is a collection of the simple (dumb) panels that
+  requires only a single power-supply.
+  There are optionally a backlight and an enable GPIO.
+  The panel may use an OF graph binding for the association to the display,
+  or it may be a direct child node of the display.
+
+  If the panel is more advanced a dedicated binding file is required.
+
+allOf:
+  - $ref: panel-common.yaml#
+
+properties:
+
+  compatible:
+    enum:
+    # compatible must be listed in alphabetical order, ordered by compatible.
+    # The description in the comment is mandatory for each compatible.
+
+        # Ampire AM-480272H3TMQW-T01H 4.3" WQVGA TFT LCD panel
+      - ampire,am-480272h3tmqw-t01h
+        # Ampire AM-800480R3TMQW-A1H 7.0" WVGA TFT LCD panel
+      - ampire,am800480r3tmqwa1h
+
+  backlight: true
+  enable-gpios: true
+  port: true
+  power-supply: true
+
+additionalProperties: false
+
+required:
+  - compatible
+  - power-supply
+
+examples:
+  - |
+    panel_rgb: panel-rgb {
+      compatible = "ampire,am-480272h3tmqw-t01h";
+      power-supply = <&vcc_lcd_reg>;
+
+      port {
+        panel_in_rgb: endpoint {
+          remote-endpoint = <&ltdc_out_rgb>;
+        };
+      };
+    };