spl: MMC U-Boot image load from raw partition

Raw images of U-Boot can be stored inside MMC partitions, so it makes sense to
read the partition table, looking for a partition number instead of using
a fixed sector address.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@ti.com>
[trini: Only add mmc_load_image_raw_partition() when
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION to avoid warning, add missing
conversion in spl_mmc_load_image()]
Signed-off-by: Tom Rini <trini@ti.com>