drm: rcar-du: Document the vsps property in the DT bindings

The property is used by the driver but is missing from the DT bindings.
Document it.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
1 file changed