media: staging: media: imx: imx7-media-csi: Increase video mem limit

Some high resolution (like Sony IMX492 47Mpix) sensors requre large amount space
for buffering.  64MB is far from sufficient so this patch increases the limit to
512MB.

Signed-off-by: Petko Manolov <petko.manolov@konsulko.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>