media: staging: ipu3-imgu: clarify the limitation of grid config

There are some grid configuration limitations for ImgU, which was
not described clearly in current uAPI header file, add the description
to help user to set the grid configuration correctly.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>