media: v4l: vsp1: Document the vsp1_du_atomic_config structure

The structure is used in the API that the VSP1 driver exposes to the DU
driver. Documenting it is thus important.

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