gpiolib: of: Prepare of_gpiochip_add() / of_gpiochip_remove() for fwnode

GPIO library is getting rid of of_node, fwnode should be utilized instead.
Prepare of_gpiochip_add() / of_gpiochip_remove() for fwnode.

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