gpio: uniphier: convert to SPDX License Identifier

checkpatch.pl suggests to use SPDX license tag. I am happy to
follow it.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-uniphier.c

-/*
- * Copyright (C) 2017 Socionext Inc.
- *   Author: Masahiro Yamada <yamada.masahiro@socionext.com>
- *
- * 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.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- */
+// SPDX-License-Identifier: GPL-2.0
+//
+// Copyright (C) 2017 Socionext Inc.
+//   Author: Masahiro Yamada <yamada.masahiro@socionext.com>
 
 #include <linux/bits.h>
 #include <linux/gpio/driver.h>