Implement defaults for the raw partition image loading so that the #ifdef
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION in spl_mmc_load_image() will no
longer be necessary.

This change makes it possible for mmc_load_image_raw_partition() and
mmc_load_image_raw_sector() to coexist.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>