soc/intel/skylake: Enable power button SMI when jumping to payload

Instead of enabling power button SMI unconditionally, add a boot state
handler to enable power button SMI just before jumping to
payload. This ensures that:
1. We do not respond to power button SMI until we know that coreboot
is done.
2. On resume, there is no need to enable power button SMI. This avoids
any power button presses during resume path from triggering a
shutdown.

BUG=b:64811381

Change-Id: Icc52dc0103555602c23e09660bc38bb4bfddbc11
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/21082
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>

src/soc/intel/skylake/pmutil.c

@@ -142,6 +142,22 @@ void enable_pm1(u16 events)
 	outw(events, ACPI_BASE_ADDRESS + PM1_EN);
 }
 
+/*
+ * Update supplied events in PM1_EN register. This does not disable any already
+ * set events.
+ */
+void update_pm1_enable(u16 events)
+{
+	u16 pm1_en = read_pm1_enable();
+	pm1_en |= events;
+	enable_pm1(pm1_en);
+}
+
+/* Read events set in PM1_EN register. */
+uint16_t read_pm1_enable(void)
+{
+	return inw(ACPI_BASE_ADDRESS + PM1_EN);
+}
 
 /*
  * SMI