gpio: remove iop driver

The iop32x platform was removed, and its gpio driver is now
orphaned.

Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>