[IA64] Clear pending interrupts at CPU boot up time

The pending interrupts can be remaining at boot up time on some
platform. This will cause spurious interrupts when interrupt is
enabled for the first time. This patch clears IVR at the CPU
initialization to eliminate such spurious interrupts.

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