drm: rcar-du: disable dtc graph-endpoint warnings on DT overlays

The rcar DT overlays are missing symetrical remote-endpoint properties
in their graph nodes because the remote-endpoint is fixed up at
run-time. Disable the dtc 'graph-endpoint' warnings when compiling these
overlays. If this becomes a common problem for overlays, then perhaps
this check needs to be disabled for all overlays.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>