mvebu: fix WRT1900AC ubinized images

The current pxa3xx_nand driver doesn't support sub-page writing,
so the VID header offset must be specified explicitly.

Signed-off-by: Claudio Leite <leitec@staticky.com>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>

SVN-Revision: 44455