[IA64] Tollhouse HP: IA64 arch changes

arch/ia64/sn and include/asm-ia64/sn changes required to support Tollhouse
system PCI hotplug, fixes the ia64_sn_sysctl_ioboard_get call, and introduces
the PRF_HOTPLUG_SUPPORT feature bit.

Signed-off-by: Prarit Bhargava <prarit@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>