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/drivers/mtd/spi/spi_flash.c b/drivers/mtd/spi/spi_flash.c
index 0c83231..25346a4 100644
--- a/drivers/mtd/spi/spi_flash.c
+++ b/drivers/mtd/spi/spi_flash.c
@@ -2,6 +2,7 @@
  * SPI flash interface
  *
  * Copyright (C) 2008 Atmel Corporation
+ * Licensed under the GPL-2 or later.
  */
 
 #include <common.h>