gpio: constify of_phandle_args in of_find_gpio_device_by_xlate()

Pointer to the struct of_phandle_args can be made const after
gpio_device_find() arguments got constified.  This should be part of
commit 4a92857d

 ("gpio: constify opaque pointer "data" in
gpio_device_find()").

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>