gpiolib: of: factor out conversion from OF flags

There are several places where we need to convert from OF flags to
"normal" GPIO_* flags, so let's introduce a helper and use it.

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