[IA64] fix NULL pointer in ia64/irq_chip-mask/unmask function

This patch fixes boot failure because irq_desc->mask() is NULL.

- Added mask/unmask functions to ia64's irq desc function table.
- rename hw_interrupt_type to irq_chip. hw_interrupt_type is old name.
- Tony: Added same change to arch/ia64/sn/kernel/irq.c as pointed out
  by Eric Biederman ... mask/unmask functions there can be no-op.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>