SFI: add platform-independent core support

drivers/sfi/sfi_core.c contains the generic SFI implementation.
It has a private header, sfi_core.h, for its own use and the
private use of future files in drivers/sfi/

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>