gpio: madera: Fixup SPDX headers

GPL-2.0-only is the preferred way of expressing v2 of the GPL, so switch
to that and remove some redundant copyright notices.
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpio-madera.c

-// SPDX-License-Identifier: GPL-2.0
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * GPIO support for Cirrus Logic Madera codecs
  *
  * Copyright (C) 2015-2018 Cirrus Logic
- *
- * 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/gpio/driver.h>