spl: spl_mmc: MMC boot mode provisions checks

This allows using only one of either raw or fs mode for SPL mmc boot, without
the need to have provisions for the other. In particular, a device may have
U-Boot installed on a file system on the mmc, without ever needing to read
U-Boot from raw memory. Thus, there is no reason to provide a sector or
partition for raw mode. This allows this behaviour and still provides a robust
fallback mechanism in case provisions for both modes are defined.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>