broadwell: Clear pending GPE events before entering sleep state

In the case of an EC wake event that is pending but not cleared
it is possible for the EC wake pin (i.e. GPIO27) to be asserted
after the kernel triggers the sleep SMI but before the system
goes to sleep.

If this happens then the GPE will be reported as a wake source
when the system wakes up again.

BUG=chrome-os-partner:33218
BRANCH=samus,auron
TEST=build and boot on samus, use the keyboard to enter suspend
with suspend_stress_test and ensure that only the RTC is listed
as a wake source upon resume.

Change-Id: I319dc22e21126a3086415f8f8b2b35eaec66fd50
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/225540
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/broadwell/smihandler.c

@@ -145,6 +145,9 @@ static void southbridge_smi_sleep(void)
 		elog_add_event_byte(ELOG_TYPE_ACPI_ENTER, slp_typ-2);
 #endif
 
+	/* Clear pending GPE events */
+	clear_gpe_status();
+
 	/* Next, do the deed.
 	 */