gpio: mockup: sort headers alphabetically

For consistency and easier maintenance: sort the headers alphabetically.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200114150253.28716-3-brgl@bgdev.pl


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