dt-bindings: display: Convert common panel bindings to DT schema

Convert the common panel bindings to DT schema consolidating scattered
definitions to a single schema file.

The 'simple-panel' binding just a collection of properties and not a
complete binding itself. All of the 'simple-panel' properties are
covered by the panel-common.txt binding with the exception of the
'no-hpd' property, so add that to the schema.

As there are lots of references to simple-panel.txt, just keep the file
with a reference to common.yaml for now until all the bindings are
converted.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705164221.4462-2-robh@kernel.org