dt-bindings: display: Convert raspberrypi,7inch-touchscreen panel to DT schema

Convert the raspberrypi,7inch-touchscreen panel binding to DT schema.

Cc: Eric Anholt <eric@anholt.net>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705164221.4462-8-robh@kernel.org