[IA64] Disable/re-enable CPE interrupts on Altix

When the CPE handler encounters too many CPEs (such as a solid single
bit memory error), it sets up a polling timer and disables the CPE
interrupt (to avoid excessive overhead logging the stream of single
bit errors).  disable_irq_nosync() calls chip->disable() to provide
a chipset specifiec interface for disabling the interrupt.  This patch
adds the Altix specific support to disable and re-enable the CPE interrupt.

Signed-off-by: Russ Anderson <(rja@sgi.com)>
Signed-off-by: Tony Luck <tony.luck@intel.com>