ARM: ixp4xx: fix gpio rework

Commit 098e30f6 "ARM: ixp4xx: stop broadcasting the custom GPIO API"
changed the internal gpio code of ixp4xx to be accessible only from
common.c, but unfortunately that broke the Goramo MultiLink code, which
uses this API.

This tries to restore the previous state without exposing the
API globally again. A better solution might be needed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Cc: Imre Kaloz <kaloz@openwrt.org>