spl: mmc: remove #ifdef CONFIG_SPL_OS_BOOT check

Implement default versions of falcon mode functions to make the
CONFIG_SPL_OS_BOOT check in spl_mmc_load_image() unnecessary, thus reducing
its #ifdef complexity.

No functional changes.

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>
Cc: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Suriyan Ramasami <suriyan.r@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>