spl: fit: Pass FIT context via a structure pointer

Several loose arguments describe the FIT image. They are thus related,
and it makes sense to pass them together, in a structure. Examples
include the FIT blob pointer, offset to FDT nodes, and the offset to
external data.

Use a spl_fit_info structure to group these parameters.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>