xen/arm,arm64: update xen_restart after ff701306 and 7b6d864b

Commit 7b6d864b (reboot: arm: change reboot_mode to use enum
reboot_mode) and ff701306 (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>