gpio: viperboard: Cut down on boilerplate

Just use the SPDX header for the license.

Cc: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-viperboard.c

+// SPDX-License-Identifier: GPL-2.0+
 /*
  *  Nano River Technologies viperboard GPIO lib driver
  *
  *  (C) 2012 by Lemonage GmbH
  *  Author: Lars Poeschel <poeschel@lemonage.de>
  *  All rights reserved.
- *
- *  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/kernel.h>