Update Linux to v5.4.2

Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/arch/x86/pci/xen.c b/arch/x86/pci/xen.c
index 9112d1c..91220cc 100644
--- a/arch/x86/pci/xen.c
+++ b/arch/x86/pci/xen.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Xen PCI - handle PCI (INTx) and MSI infrastructure calls for PV, HVM and
  * initial domain support. We also handle the DSDT _PRT callbacks for GSI's