Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/include/linux/mailbox_client.h b/include/linux/mailbox_client.h
index 4434871..65229a4 100644
--- a/include/linux/mailbox_client.h
+++ b/include/linux/mailbox_client.h
@@ -1,10 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Copyright (C) 2013-2014 Linaro Ltd.
  * Author: Jassi Brar <jassisinghbrar@gmail.com>
- *
- * 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.
  */
 
 #ifndef __MAILBOX_CLIENT_H
@@ -44,6 +41,7 @@
 					      const char *name);
 struct mbox_chan *mbox_request_channel(struct mbox_client *cl, int index);
 int mbox_send_message(struct mbox_chan *chan, void *mssg);
+int mbox_flush(struct mbox_chan *chan, unsigned long timeout);
 void mbox_client_txdone(struct mbox_chan *chan, int r); /* atomic */
 bool mbox_client_peek_data(struct mbox_chan *chan); /* atomic */
 void mbox_free_channel(struct mbox_chan *chan); /* may sleep */