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>



git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44455 3c298f89-4303-0410-b956-a3cf2f4a3e73