Commit 7b6d864b48 (reboot: arm: change reboot_mode to use enum
reboot_mode) and ff701306cd (arm64: use common reboot infrastructure)
change the prototype of arm_pm_restart on arm and arm64.
Update xen_restart accordingly.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>