spl: Add an arch-specific hook for writing to SPL handoff

At present there is an arch-specific area in the SPL handoff area intended
for use by arch-specific code, but there is no explicit call to fill in
this data. Add a hook for this.

Also use the hook to remove the sandbox-specific test code from
write_spl_handoff().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>