soc/intel/common/block/smm: Add mainboard_smi_finalize

This commit adds a method called `mainboard_smi_finalize` which provides
a mechanism for a mainboard to execute some code as part of the finalize
method in the SMM stage before SoC does its finalization.

BUG=b:191189275
BRANCH=None
TEST=Implement `mainboard_smi_finalize` on lalala and verify that the
code executes in SMM.

Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Change-Id: If1ee63431e3c2a5831a4656c3a361229acff3f42
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55649
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>

src/include/cpu/x86/smm.h

@@ -49,6 +49,7 @@ void southbridge_smi_handler(void);
 void mainboard_smi_gpi(u32 gpi_sts);
 int  mainboard_smi_apmc(u8 data);
 void mainboard_smi_sleep(u8 slp_typ);
+void mainboard_smi_finalize(void);
 
 /* This is the SMM handler. */
 extern unsigned char _binary_smm_start[];