ARM: ixp4xx: stop broadcasting the custom GPIO API

Now that these custom GPIO accessors are only used from the
gpio chip in this machine, move the code out of the include
file and right next to the gpiochip implementation.

Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>