gpio: ath79: add missing header

Include mod_devicetable.h for struct of_device_id and its helpers.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240821121456.19553-2-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>