[media] s5p-mfc: Reduce firmware buffer size for MFC v6+ variants

Firmware for MFC v6+ variants is not larger than 400 KiB, so there is no
need to allocate a full 1 MiB buffer for it. Reduce it to 512 KiB to keep
proper alignment of allocated buffer.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Tested-by: Smitha T Murthy <smitha.t@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>