xen/arm: Define xen_arch_suspend()

Commit 2b953a5e ("xen: Suspend ticks on all CPUs during suspend")
introduced xen_arch_suspend() routine but did so only for x86, breaking
ARM builds.

We need to add it to ARM as well.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reported-by: Michal Suchanek <hramrach@gmail.com>
Tested-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>