linux-phytec-ti

gpio: mockup: use the SPDX identifier and remove license boilerplate

Use the SPDX license identifier for GPLv2.0 or later and remove the
license boilerplate.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>