um: Add SPDX headers for files in arch/um/drivers

Convert files to use SPDX header. All files are licensed under the GPLv2.

Signed-off-by: Alex Dewar <alex.dewar@gmx.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/ubd_user.c

@@ -1,8 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2016 Anton Ivanov (aivanov@brocade.com)
  * Copyright (C) 2000, 2001, 2002 Jeff Dike (jdike@karaya.com)
  * Copyright (C) 2001 Ridgerun,Inc (glonnon@ridgerun.com)
- * Licensed under the GPL
  */
 
 #include <stddef.h>