spl_mmc: add Falcon mode support for raw variant

If Falcon mode support is enabled (and the system isn't directed into
booting u-boot), it will instead try to load kernel from sector
CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR and
CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTORS of kernel argument parameters
starting from sector CONFIG_SYS_MMCSD_RAW_MODE_ARGS_SECTOR.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>