xen: Introduce xen_pfn_t for pfn and mfn types

All the original Xen headers have xen_pfn_t as mfn and pfn type, however
when they have been imported in Linux, xen_pfn_t has been replaced with
unsigned long. That might work for x86 and ia64 but it does not for arm.
Bring back xen_pfn_t and let each architecture define xen_pfn_t as they
see fit.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>