sf: add GPL-2 license info

Some of the new spi flash files were missing explicit license lines.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
diff --git a/common/cmd_sf.c b/common/cmd_sf.c
index d571f60..d69ae6a 100644
--- a/common/cmd_sf.c
+++ b/common/cmd_sf.c
@@ -2,7 +2,9 @@
  * Command for accessing SPI flash.
  *
  * Copyright (C) 2008 Atmel Corporation
+ * Licensed under the GPL-2 or later.
  */
+
 #include <common.h>
 #include <spi_flash.h>