drm: rcar-du: kms: Expand comment in vsps parsing routine

Expand comment in the 'vsps' parsing routine to specify the LIF
channel index defaults to 0 in case the second cell of the property
is not specified to remain compatible with older DT bindings.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>