cpu/x86: Only include smm code if CONFIG_HAVE_SMI_HANDLER=y

This removes the need to include this code separately on each
platform.

Change-Id: I3d848b1adca4921d7ffa2203348073f0a11d090e
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46380
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/soc/intel/skylake/Makefile.inc

@@ -7,7 +7,6 @@ subdirs-y += ../../../cpu/intel/microcode
 subdirs-y += ../../../cpu/intel/turbo
 subdirs-y += ../../../cpu/x86/lapic
 subdirs-y += ../../../cpu/x86/mtrr
-subdirs-y += ../../../cpu/x86/smm
 subdirs-y += ../../../cpu/x86/tsc
 
 bootblock-y += bootblock/bootblock.c