x86: Adjust the FSP types slightly

To avoid casts, find_fsp_header() should return a pointer. Add asmlinkage
to two API functions which use that convention. UPD_TERMINATOR is common
so move it into a common file.

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