mailbox: bcm2835: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.

Cc: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/bcm2835-mailbox.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  *  Copyright (C) 2010,2015 Broadcom
  *  Copyright (C) 2013-2014 Lubomir Rintel
  *  Copyright (C) 2013 Craig McGeachie
  *
- * 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 device provides a mechanism for writing to the mailboxes,
- * that are shared between the ARM and the VideoCore processor
- *
  * Parts of the driver are based on:
  *  - arch/arm/mach-bcm2708/vcio.c file written by Gray Girling that was
  *    obtained from branch "rpi-3.6.y" of git://github.com/raspberrypi/