hw/arm: Use GED for system_powerdown event

hw/arm: Use GED for system_powerdown event

For machines 4.2 or higher with ACPI boot use GED for system_powerdown
event instead of GPIO. Guest boot with DT still uses GPIO.

Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190918130633.4872-9-shameerali.kolothum.thodi@huawei.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>