spl: Add default values for ARCH_MX7

ARCH_MX6 has default values for SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR
and SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR.

Do the same for ARCH_MX7 so that users may have a consistent
experience through the i.MX families.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>