gpio: ath79: use generic device property getters

Don't use specialized OF accessors if we can avoid it: switch to using
the generic device property helpers.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240821121456.19553-3-brgl@bgdev.pl


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