spl: Convert nor to spl_load

This converts the nor load method to use spl_load. As a result it also
adds support for LOAD_FIT_FULL. Since this is the last caller of
spl_load_legacy_img, it has been removed.

We can't load FITs with external data with SPL_LOAD_FIT_FULL, so disable the
test in that case. No boards enable SPL_NOR_SUPPORT and SPL_LOAD_FIT_FULL, so
this is not a regression.

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