[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>

arch/ia64/kernel/irq.c

@@ -33,6 +33,11 @@ void ack_bad_irq(unsigned int irq)
 }
 
 #ifdef CONFIG_IA64_GENERIC
+ia64_vector __ia64_irq_to_vector(int irq)
+{
+	return irq_cfg[irq].vector;
+}
+
 unsigned int __ia64_local_vector_to_irq (ia64_vector vec)
 {
 	return __get_cpu_var(vector_irq)[vec];