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.pl


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>