xen/arm: account for stolen ticks

Register the runstate_memory_area with the hypervisor.
Use pv_time_ops.steal_clock to account for stolen ticks.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>


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Changes in v4:
- don't use paravirt_steal_rq_enabled: we do not support retrieving
stolen ticks for vcpus other than one we are running on.

Changes in v3:
- use BUG_ON and smp_processor_id.