drm/panel: Device tree bindings for ARM Versatile panels

This adds a pretty simple set of device tree bindings for
ARM Versatile panels appearing as child nodes of a system
controller.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205192013.5349-1-linus.walleij@linaro.org