hw/arm: Factor out powerdown notifier from GPIO

hw/arm: Factor out powerdown notifier from GPIO

This is in preparation of using GED device for
system_powerdown event. Make the powerdown notifier
registration independent of create_gpio() fn.

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: <20190918130633.4872-8-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>