Add GPL-2.0+ SPDX-License-Identifier to source files

Signed-off-by: Wolfgang Denk <wd@denx.de>
[trini: Fixup common/cmd_io.c]
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/include/linker_lists.h b/include/linker_lists.h
index 6c28bf9..1eebb95 100644
--- a/include/linker_lists.h
+++ b/include/linker_lists.h
@@ -5,13 +5,7 @@
  *
  * Copyright (C) 2012 Marek Vasut <marex@denx.de>
  *
- * See file CREDITS for list of people who contributed to this
- * project.
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as
- * published by the Free Software Foundation; either version 2 of
- * the License, or (at your option) any later version.
+ * SPDX-License-Identifier:	GPL-2.0+
  */
 
 /*