pwm: bcm2835: Switch to SPDX identifier

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

Cc: Bart Tanghe <bart.tanghe@thomasmore.be>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-bcm2835.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright 2014 Bart Tanghe <bart.tanghe@thomasmore.be>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; version 2.
  */
 
 #include <linux/clk.h>