media: vsp1: Document vsp1_dl_body refcnt

In commit 2d9445db ("media: vsp1: Use reference counting for
bodies"), a new field was introduced to the vsp1_dl_body structure to
account for usage tracking of the body.

Document the newly added field in the kerneldoc.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>