drm: rcar-du: Recognize "renesas,vsps" in addition to "vsps"

The Renesas-specific "vsps" property lacks a vendor prefix.
Add a "renesas," prefix to comply with DT best practises.
Retain backward compatibility with old DTBs by falling back to "vsps"
when needed.

Fixes: 6d62ef3a

 ("drm: rcar-du: Expose the VSP1 compositor through KMS planes")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>