x86: add arch-specific SFI support

arch/x86/kernel/sfi.c serves the dual-purpose of supporting the
SFI core with arch specific code, as well as a home for the
arch-specific code that uses SFI.

analogous to ACPI, drivers/sfi/Kconfig is pulled in by arch/x86/Kconfig

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org