ARM: Virt: Add a GPIO controller

ACPI 5.0 supports GPIO-signaled ACPI Events. This can be used for
powerdown, hotplug evnets. Add a GPIO controller in machine virt,
to support powerdown, maybe can be used for cpu hotplug. And
here we use pl061.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Wei Huang <wei@redhat.com>
Tested-by: Wei Huang <wei@redhat.com>
Message-id: 1449804086-3464-4-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>