x86: provide infrastructure to backup default SMM region

Certain CPUs require the default SMM region to be backed up
on resume after a suspend. The reason is that in order to
relocate the SMM region the default SMM region has to be used.
As coreboot is unaware of how that memory is used it needs to
be backed up. Therefore provide a common method for doing this.

Change-Id: I65fe1317dc0b2203cb29118564fdba995770ffea
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5216
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/include/cpu/x86/smm.h

@@ -571,4 +571,8 @@ int smm_load_module(void *smram, int size, struct smm_loader_params *params);
 #endif /* __SMM__ */
 #endif /* CONFIG_SMM_MODULES */
 
+/* Backup and restore default SMM region. */
+void *backup_default_smm_area(void);
+void restore_default_smm_area(void *smm_save_area);
+
 #endif