dtc: Replace GPLv2 boilerplate/reference with SPDX tags

Replace instances of GPLv2 or later boilerplate with SPDX tags.

Signed-off-by: Rob Herring <robh@kernel.org>
Message-Id: <20190620211944.9378-2-robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/Makefile.dtc b/Makefile.dtc
index d437563..9c467b0 100644
--- a/Makefile.dtc
+++ b/Makefile.dtc
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
 # Makefile.dtc
 #
 # This is not a complete Makefile of itself.  Instead, it is designed to