spl: spl-nor: return error if no valid image was loaded

If only FIT images are enabled and loading the FIT image fails,
spl_nor_load_image() should return an error instead of zero.

Without this patch:

>>SPL: board_init_r()
spl_init
Trying to boot from NOR
Unsupported OS image.. Jumping nevertheless..
image entry point: 0x0

With patch:

>>SPL: board_init_r()
spl_init
Trying to boot from NOR
SPL: failed to boot from all boot devices (err=-6)
.### ERROR ### Please RESET the board ###

Signed-off-by: Mario Kicherer <dev@kicherer.org>