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/lib/asm-offsets.c b/lib/asm-offsets.c
index 8546a52..6ea7b03 100644
--- a/lib/asm-offsets.c
+++ b/lib/asm-offsets.c
@@ -9,10 +9,7 @@
  * compile this file to assembler, and then extract the
  * #defines from the assembly-language output.
  *
- * 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+
  */
 
 #include <common.h>