gpio: ep93xx: Switch to SPDX license tag

The subject says it all. Cut down on boilerplate.
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-ep93xx.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * Generic EP93xx GPIO handling
  *
@@ -6,10 +7,6 @@
  *
  * Based on code originally from:
  *  linux/arch/arm/mach-ep93xx/core.c
- *
- *  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.
  */
 
 #include <linux/init.h>