Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/c6x/lib/Makefile b/arch/c6x/lib/Makefile
index ffd3c65..e182004 100644
--- a/arch/c6x/lib/Makefile
+++ b/arch/c6x/lib/Makefile
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 #
 # Makefile for arch/c6x/lib/
 #