[IA64] SN fix for cpu hotplug/kexec

The sn_cpu_init() is required for cpu initialization on SN platforms.
Change __init to __cpuinit so that the function is not freed with init code/data.

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