gpiolib: of: factor out code overriding gpio line polarity

There are several instances where we use a separate property to
override polarity specified in gpio property. Factor it out into
a separate function.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>