armv7: ls1021aiot: Fixing SPL compiling issues

To align with SPL change 38fed8ab and 693d4c9f, add Kconfig option
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR to defconfig, and remove
CONFIG_SYS_U_BOOT_MAX_SIZE_SECTORS.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Feng Li <feng.li_2@nxp.com>