gpio: mockup: update the license tag

The current GPL v2.0 or later SPDX tag is 'GPL-2.0-or-later' as defined
at https://spdx.org/licenses/.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200114150253.28716-2-brgl@bgdev.plSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-mockup.c

-// SPDX-License-Identifier: GPL-2.0+
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
  * GPIO Testing Device Driver
  *