imx: mx6q/dl sabre_common: Move MMC ENV offset to 512KB position

Set the CONFIG_ENV_OFFSET from 384KB to 512KB offset, so we will have larger space
for u-boot image.
When enabling NAND flash support feature, the u-boot image size has exceeded the
384KB, which causes overlay to the environment variables storage.

Signed-off-by: Ye.Li <B37916@freescale.com>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>