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/include/cpu/x86/smm.h

@@ -272,3 +272,4 @@ void __attribute__((weak)) southbridge_smi_handler(unsigned int node, smm_state_
 
 void __attribute__((weak)) mainboard_smi_gpi(u16 gpi_sts);
 int __attribute__((weak)) mainboard_apm_cnt(u8 data);
+void smi_release_lock(void);