Documentation: gpio: driver.rst: Fix warnings

Fix warnings due to incorrect rst markup. Also improved the presentation
a little without changing the underlying content.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20191122034702.58563-1-dwlsalmeida@gmail.com


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