MIPS: ath79: fix GPIO function selection for AR934x SoCs

GPIO function selection is not working on the AR934x
SoCs because the offset of the function selection
register is different on those.

Add a helper routine which returns the correct
register address based on the SoC type, and use
that in the 'ath79_gpio_function_*' routines.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4870/
Signed-off-by: John Crispin <blogic@openwrt.org>