xen/arm: SMP support

Map vcpu_info using VCPUOP_register_vcpu_info on all the online vcpus,
make sure the allocated struct doesn't cross a page boundary.

Call enable_percpu_irq on every cpu.

Changes in v5:
- allocate xen_vcpu_info dynamically, aligning it to the size of the
struct;
- use VCPUOP_register_vcpu_info on cpu0 too.

Changed in v2:
- move the percpu variable argument fix to a separate patch;
- remove unused variable.

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