drivers: gpio: Kconfig: pedantic formatting cleanups

Align the Kconfig formatting with the vast majority of the Kconfig
files, to make it a bit easier / more pleasant to read ;-)

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>