gpio: mockup: use the SPDX identifier and remove license boilerplate

Use the SPDX license identifier for GPLv2.0 or later and remove the
license boilerplate.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-mockup.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  * GPIO Testing Device Driver
  *
  * Copyright (C) 2014  Kamlakant Patel <kamlakant.patel@broadcom.com>
  * Copyright (C) 2015-2016  Bamvor Jian Zhang <bamv2005@gmail.com>
  * Copyright (C) 2017 Bartosz Golaszewski <brgl@bgdev.pl>
- *
- * 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;  either version 2 of the  License, or (at your
- * option) any later version.
- *
  */
 
 #include <linux/init.h>