Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/x86/xen/pci-swiotlb-xen.c b/arch/x86/xen/pci-swiotlb-xen.c
index 37c6056..33293ce 100644
--- a/arch/x86/xen/pci-swiotlb-xen.c
+++ b/arch/x86/xen/pci-swiotlb-xen.c
@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-2.0
+
 /* Glue code to lib/swiotlb-xen.c */
 
 #include <linux/dma-mapping.h>