spl: mmc: move fs boot into its own function

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 <[email protected]>
Cc: Igor Grinberg <[email protected]>
Cc: Paul Kocialkowski <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Simon Glass <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Tom Rini <[email protected]>