SMM: don't overwrite SMM memory on resume

Overwriting the SMM Area on resume leaves us with
all variables cleared out, i.e., the GNVS pointer
is no longer available, which makes SMIF function
calls impossible.

Change-Id: I08ab4ffd41df0922d63c017822de1f89a3ff254d
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/34
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/cpu/x86/smm/smihandler.c

@@ -46,7 +46,7 @@ static int smi_obtain_lock(void)
 	return (ret == SMI_UNLOCKED);
 }
 
-static void smi_release_lock(void)
+void smi_release_lock(void)
 {
 	asm volatile (
 		"movb %1, %%al\n"