dt-bindings: display: add panel-timing.yaml

Add meta-schema variant of panel-timing and
reference it from panel-common.yaml.

Part of this came form other files with other
licenses - original commits:

commit cc3f414c ("video: add of helper for display timings/videomode")
commit 86f46565 ("dt-bindings: display: display-timing: Add property to configure sync drive edge")
commit 9cad9c95

 ("Documentation: DocBook DRM framework documentation")

The original authors acked the license change to:
(GPL-2.0-only OR BSD-2-Clause)

v2:
  - Got OK from original authors for re-license
    Huge thanks for the quick replies!
  - Typo fixes (Oleksandr)
  - Drop -array variant when not needed (Maxime)
  - Replace oneOf:... with enum (Maxime)
  - Drop type from clock-frequency (Rob)
  - Drop "|" when not needed (Rob)

v3:
  - Added comment to acks that are only for the license change
  - Add yaml document terminator "..."
  - Updated description (removed reference to native-mode)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> [license change]
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com> [license change]
Acked-by: Steffen Trumtrar <s.trumtrar@pengutronix.de> [license change]
Acked-by: Philipp Zabel <p.zabel@pengutronix.de> [license change]
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: devicetree@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200216181513.28109-2-sam@ravnborg.org