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

The opaque pointer "data" in each match function used by
gpio_device_find() is a pointer to const, thus the same argument passed
to gpio_device_find() can adjusted similarly.

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