mx6sabre_common: Fix U-Boot corruption after 'saveenv'

Booting mx6qp sabreauto board and then doing:

=> saveenv
=> reset

, causes a system hang.

This happens because the size of the U-Boot binary is larger than
CONFIG_ENV_OFFSET.

Fix this problem by increasing CONFIG_ENV_OFFSET, so that the U-boot binary
and the environment variables region do not overlap.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>