mb/google/volteer: Add EC_HOST_EVENT_USB_MUX

This changes adds the EC_HOST_EVENT_USB_MUX to be dark resume source.

BUG=b:183140386
TEST=In S0ix, remove DP dongle, system does dark resume. AP and EC
synchronized. AP got port partner disconnection.

Signed-off-by: John Zhao <john.zhao@intel.com>
Change-Id: I53bd4fee21e2e2d1f16f558ab0341a50ef9a0e14
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52716
Reviewed-by: Tanu Malhotra <tanu.malhotra@intel.com>
Reviewed-by: Abe Levkoy <alevkoy@chromium.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/google/volteer/variants/baseboard/include/baseboard/ec.h b/src/mainboard/google/volteer/variants/baseboard/include/baseboard/ec.h
index 1428ec8..253ab68 100644
--- a/src/mainboard/google/volteer/variants/baseboard/include/baseboard/ec.h
+++ b/src/mainboard/google/volteer/variants/baseboard/include/baseboard/ec.h
@@ -37,6 +37,7 @@
 	 EC_HOST_EVENT_MASK(EC_HOST_EVENT_AC_CONNECTED)    |\
 	 EC_HOST_EVENT_MASK(EC_HOST_EVENT_AC_DISCONNECTED) |\
 	 EC_HOST_EVENT_MASK(EC_HOST_EVENT_KEY_PRESSED)     |\
+	 EC_HOST_EVENT_MASK(EC_HOST_EVENT_USB_MUX)         |\
 	 EC_HOST_EVENT_MASK(EC_HOST_EVENT_MODE_CHANGE))
 
 #define MAINBOARD_EC_S0IX_WAKE_EVENTS	(MAINBOARD_EC_S3_WAKE_EVENTS)