ARM: ux500: allow Snowball access to the AB8500 GPIO pins

The AB8500 GPIO driver has been un-BROKEN and rewritten as a pinctrl
driver. Now that it's back in use, let's ensure that it's available
when booting Snowball with Device Tree enabled.

Cc: arm@kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>