Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/drivers/mailbox/mailbox.h b/drivers/mailbox/mailbox.h
index 456ba68..4e3cc44 100644
--- a/drivers/mailbox/mailbox.h
+++ b/drivers/mailbox/mailbox.h
@@ -1,8 +1,4 @@
-/*
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- */
+/* SPDX-License-Identifier: GPL-2.0-only */
 
 #ifndef __MAILBOX_H
 #define __MAILBOX_H