gpio: xgene: Enable X-Gene standby GPIO as interrupt controller

Enable X-Gene standby GPIO controller as interrupt controller to provide
its own resources. This avoids ambiguity where GIC interrupt resource is
use as X-Gene standby GPIO interrupt resource in user driver.
Signed-off-by: Y Vo <yvo@apm.com>
Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>