imx: mx6sxsabresd: enlarge ENV offset

The u-boot-dtb.imx size is about 519KB, so 8 * 64KB conflicts
with u-boot-dtb.imx. Enlarge the offset to 14 * 64KB to fix it.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>