gpio: of: replace gpiochip_find_* with gpio_device_find_*

We're porting all users of gpiochip_find() to using gpio_device_find().
Update the OF GPIO code.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>