[IA64] SN2 needs platform specific irq_to_vector() function.

Add base support for implementing platform_irq_to_vector(), and
then use it on SN2.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Acked-by: John Keller <jpk@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>