gpio: ath79: use devm_platform_ioremap_resource()

There's no need to use the nocache variant of ioremap(). Switch to
using devm_platform_ioremap_resource().

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>