MIPS: Ath79: Remove the unused GPIO function API

To prepare moving the GPIO driver to drivers/gpio remove the
platform specific pinmux API. As it is not used by any board,
and such functionality should better be implemented using the
pinmux subsystem just removing it seems to be the best option.

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/10596/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>