spl: Convert spi to spl_load

This converts the spi load method to use spl_load. The address used for
LOAD_FIT_FULL may be different, but there are no in-tree users of that
config. Since payload_offs is only used without OS_BOOT, we defer its
initialization.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>