Instead of just depending on OF and getting build failures,
depend on ARM && OF_GPIO.

Cc: Fabian Vogt <fabian@ritter-vogt.de>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>