gpio: pl061: don't depend on CONFIG_ARM

The pl061 driver has no real dependency on ARM, so remove the kconfig
dependency.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>