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