[IA64] Increase max node count on SN platforms

Add support in IA64 acpi for platforms that support more than
256 nodes. Currently, ACPI is limited to 256 nodes because the
proximity domain number is 8-bits.

Long term, we expect to use ACPI3.0 to support >256 nodes.
This patch is an interim solution that works with platforms
that pass the  high order bits of the proximity domain in
"reserved" fields of the ACPI tables. This code is enabled
ONLY on SN platforms.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>