MIPS: ath79: Move the GPIO driver to drivers/gpio

MIPS: ath79: Move the GPIO driver to drivers/gpio

GPIO drivers should be in drivers/gpio

Signed-off-by: Alban Bedel <albeu@free.fr>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10597/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>