docs: driver-api: gpio: using-gpio.rst: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>