brcm63xx: fix A4001N image padding size

When r42245 moved A4001N to DT based detection the image padding size
was accidentally increased from 4 to 8 MiB. This made images unflashable
on these devices.

Fixes #19836.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 45945