Update license headers

Update all license headers to the correct years

Change-Id: I638a854b39b4afe8db6a4a76ca6dcb89c126e3c1
diff --git a/drivers/mailbox/src/mailbox.cpp b/drivers/mailbox/src/mailbox.cpp
index 96c80df..e5950f0 100644
--- a/drivers/mailbox/src/mailbox.cpp
+++ b/drivers/mailbox/src/mailbox.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020 Arm Limited. All rights reserved.
+ * Copyright (c) 2020-2021 Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: Apache-2.0
  *