Documentation: gpio: consistently use logical line value terminology

Consistently use active/inactive to describe logical line values, rather
than high/low, which is used for physical values, or asserted/de-asserted
which is awkward.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
[Bartosz: tweaked the commit subject to use imperative mood]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>