xen/arm: enable PV control for ARM

Enable lifecyle management (reboot, shutdown...) from the toolstack
for ARM guests.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>