spl: mmc: move fs boot into its own function

Move the code that handles fs boot out of spl_mmc_load_image() and into its
own function to reduce the #ifdef complexity of spl_mmc_load_image().

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>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>