SPL: mmc: Make spl_mmc_load_image available

Make the spl_mmc_load_image() available globally, so it can be
invoked directly by SPL on extremely space-constrained systems.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>