gpio: add gpiod_toggle_active_low()

Add possibility to toggle active-low flag of a gpio descriptor. This is
useful for compatibility code, where defaults are inverted vs DT gpio
flags or the active-low flag is taken from elsewhere.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/7ce0338e01ad17fa5a227176813941b41a7c35c1.1576031637.git.mirq-linux@rere.qmqm.pl
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>