spi: Add GPL-2.0+ SPDX-License-Identifier for missing files

Added GPL-2.0+ SPDX-License-Identifier for missed spi
source files.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
diff --git a/drivers/spi/xilinx_spi.h b/drivers/spi/xilinx_spi.h
index 69d0b94..ce7d82c 100644
--- a/drivers/spi/xilinx_spi.h
+++ b/drivers/spi/xilinx_spi.h
@@ -9,7 +9,7 @@
  * Copyright (c) 2010 Graeme Smecher <graeme.smecher@mail.mcgill.ca>
  * Copyright (c) 2012 Stephan Linz <linz@li-pro.net>
  *
- * Licensed under the GPL-2 or later.
+ * SPDX-License-Identifier:	GPL-2.0+
  *
  * [0]: http://www.xilinx.com/support/documentation
  *