ARM: dts: omap4-droid4: add panel compatible

Add Droid 4 specific compatible value in addition to the
generic one, so that we have the ability to add panel
specific quirks in the future.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>