MAINTAINERS: Add file patterns for drm device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463907991-7916-17-git-send-email-geert@linux-m68k.org

MAINTAINERS

@@ -3765,6 +3765,9 @@ T:	git git://people.freedesktop.org/~airlied/linux
 S:	Maintained
 F:	drivers/gpu/drm/
 F:	drivers/gpu/vga/
+F:	Documentation/devicetree/bindings/display/
+F:	Documentation/devicetree/bindings/gpu/
+F:	Documentation/devicetree/bindings/video/
 F:	Documentation/DocBook/gpu.*
 F:	include/drm/
 F:	include/uapi/drm/