Update license headers

Update all license headers to the correct years

Change-Id: I638a854b39b4afe8db6a4a76ca6dcb89c126e3c1
diff --git a/drivers/mailbox/CMakeLists.txt b/drivers/mailbox/CMakeLists.txt
index 9e56d37..bf8889a 100644
--- a/drivers/mailbox/CMakeLists.txt
+++ b/drivers/mailbox/CMakeLists.txt
@@ -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
 #