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>

Documentation/devicetree/bindings/display/renesas,du.txt

@@ -36,6 +36,9 @@ Required Properties:
       When supplied they must be named "dclkin.x" with "x" being the input
       clock numerical index.
 
+  - vsps: A list of phandles to the VSP nodes that handle the memory
+    interfaces for the DU channels.
+
 Required nodes:
 
 The connections to the DU output video ports are modeled using the OF graph