ARM: Virt: Add QEMU powerdown notifier and hook it to GPIO Pin 3

ARM: Virt: Add QEMU powerdown notifier and hook it to GPIO Pin 3

Currently mach-virt model doesn't support powerdown request. Guest VM
doesn't react to system_powerdown from monitor console (or QMP) because
there is no communication mechanism for such requests. This patch registers
GPIO Pin 3 with powerdown notification. So guest VM can receive notification
when such powerdown request is triggered.

Signed-off-by: Wei Huang <wei@redhat.com>
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: 1449804086-3464-10-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>