ia64/xen: increase IA64_MAX_RSVD_REGIONS.

Xenlinux/ia64 needs to reserve one more region passed from xen hypervisor
as start info.

Cc: Robin Holt <holt@sgi.com>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>