arm: mvebu: ds414: define CONFIG_SYS_U_BOOT_OFFS

Without CONFIG_SYS_U_BOOT_OFFS set to CONFIG_SYS_SPI_U_BOOT_OFFS, U-Boot
will be located at address 0x16000. But, SPL will try to load the
payload from 0x24000 causing the boot to hang.

Signed-off-by: Ezra Buehler <ezra@easyb.ch>
Reviewed-by: Stefan Roese <sr@denx.de>