gpio: ath79: remove support for platform data

There are no more board files defining platform data for this driver so
remove the header and support from the driver.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240821121456.19553-4-brgl@bgdev.pl


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