pwm: cros-ec: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-cros-ec.c

+// SPDX-License-Identifier: GPL-2.0
 /*
- * Copyright (C) 2016 Google, Inc
- *
- * 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.
- *
  * Expose a PWM controlled by the ChromeOS EC to the host processor.
+ *
+ * Copyright (C) 2016 Google, Inc.
  */
 
 #include <linux/module.h>