soc/intel/cannonlake: Enable power button smi in pre-OS

This change enables user to shutdown the system by shortly pressing
power button (<10sec) before OS is loaded. Main use case is shutdown
from recovery/broken screen.

BUG=N/A
TEST=Boot up into recovery screen on Sarien platform, press power button
once, and system should shutdown immediatelly.

Change-Id: I7655daf65ff058df7d9bad4567f74b4f4007acb4
Signed-off-by: Krzysztof Sywula <krzysztof.m.sywula@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31894
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lijian Zhao <lijian.zhao@intel.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/cannonlake/cpu.c

@@ -456,7 +456,7 @@ static void post_mp_init(void)
 	 * Now that all APs have been relocated as well as the BSP let SMIs
 	 * start flowing.
 	 */
-	smm_southbridge_enable(PWRBTN_EN | GBL_EN);
+	smm_southbridge_enable(GBL_EN);
 
 	/* Lock down the SMRAM space. */
 	smm_lock();