ACPI: Altix: ACPI _PRT support

Provide ACPI _PRT support for SN Altix systems.

The SN Altix platform does not conform to the
IOSAPIC IRQ routing model, so a new acpi_irq_model
(ACPI_IRQ_MODEL_PLATFORM) has been defined. The SN
platform specific code sets acpi_irq_model to
this new value, and keys off of it in acpi_register_gsi()
to avoid the iosapic code path.

Signed-off-by: John Keller <jpk@sgi.com>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/arch/ia64/kernel/acpi.c b/arch/ia64/kernel/acpi.c
index 73ef4a85..ef2fe47 100644
--- a/arch/ia64/kernel/acpi.c
+++ b/arch/ia64/kernel/acpi.c
@@ -590,6 +590,9 @@
  */
 int acpi_register_gsi(u32 gsi, int triggering, int polarity)
 {
+	if (acpi_irq_model == ACPI_IRQ_MODEL_PLATFORM)
+		return gsi;
+
 	if (has_8259 && gsi < 16)
 		return isa_irq_to_vector(gsi);