gpio: ath79: convert to dynamic GPIO base allocation

ath79 target has already been converted to device tree based platform.
Use dynamic GPIO numberspace base to suppress the warning:

gpio gpiochip0: Static allocation of GPIO base is deprecated, use dynamic allocation.

Tested on Atheros AR7241 and AR9344.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://lore.kernel.org/r/TYCP286MB089598EA71E964BD8AB9EFD3BCD62@TYCP286MB0895.JPNP286.PROD.OUTLOOK.COM


[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>