media: vsp1: Document max_width restriction on SRU

The SRU is currently restricted to 256 pixels as part of the current
partition algorithm. Document that the actual capability of this
component is 288 pixels, but don't increase the implementation.

The extended partition algorithm may later choose to utilise a larger
input to support overlapping partitions which will improve the quality
of the output images.

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