spl: Convert spl_fit to work with sandbox

At present this casts addresses to pointers so cannot work with sandbox.
Update the code to use map_sysmem() instead.

As part of this change, the existing load_ptr is renamed to src_ptr since
it is not a pointer to load_addr. It is confusing to use a similar name
for something that is not actually related. For the alignment code,
ALIGN() is used instead of open-coded alignment. Add a comment to the line
that casts away a const.

Use a (new) load_ptr variable to access memory at address load_addr.

Signed-off-by: Simon Glass <sjg@chromium.org>