amd: Don't call halt() when in SMM

This copies what Intel does.

BUG=b:80295434
TEST=grunt: Made sure that the S5 SMI interrupt gets fired.

Change-Id: I7874824cad01054c6bdeff12d248e671f27be030
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/26545
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/amd/common/block/acpi/halt.c

@@ -20,5 +20,12 @@
 void poweroff(void)
 {
 	outl((SLP_TYP_S5 << SLP_TYP_SHIFT) | SLP_EN, pm_acpi_pm_cnt_blk());
-	halt();
+
+	/*
+	 * Setting SLP_TYP_S5 in PM1 triggers SLP_SMI, which is handled by SMM
+	 * to transition to S5 state. If halt is called in SMM, then it prevents
+	 * the SMI handler from being triggered and system never enters S5.
+	 */
+	if (!ENV_SMM)
+		halt();
 }