gpiolib: of: factor out quirk setting polarity via separate property

Several legacy bindings use a separate property to specify polarity of
GPIOs instead of specifying it directly in the GPIO property. Factor
out this code to make it easier to add more such cases.

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