Documentation: gpio: remove duplicated lines

The 'default (active high)' lines are repeated twice. Avoid people stare at
their screens looking for differences.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>