mailbox: ti-msgmgr: Switch to SPDX Licensing

Switch to SPDX licensing and drop the GPL text which comes redundant.
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/ti-msgmgr.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Texas Instruments' Message Manager Driver
  *
- * Copyright (C) 2015-2016 Texas Instruments Incorporated - http://www.ti.com/
+ * Copyright (C) 2015-2017 Texas Instruments Incorporated - http://www.ti.com/
  *	Nishanth Menon
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
- * This program is distributed "as is" WITHOUT ANY WARRANTY of any
- * kind, whether express or implied; without even the implied warranty
- * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
  */
 
 #define pr_fmt(fmt) "%s: " fmt, __func__