dt-bindings: display: rcar-du: Add a VSP channel index to the vsps DT property

On some R-Car SoCs a single VSP can serve multiple DU channels through
multiple LIF instances in the VSP. The current DT bindings don't support
specifying that kind of SoC integration scheme. Extend them with a VSP
channel index.

Backward compatibility can be ensured in drivers by checking the length
of the vsps property and setting the channel to 0 when the property
doesn't contain channel indices.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>