[IA64] Increase max NR_CPUS on IA64 from 512 to 1024

This patch increases the maximum number of cpus supported on IA64
to 1024. No changes are made to the default SSI size. The patch
simply allows specifying up to 1024p.

There are certainly scaling (& other) issues that also need to be
addressed!!!  Additional patches will follow.....

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