Introduce struct ioapic with nr_registers field.

This will pave way for consolidating different MAX_IO_APICS
arrays into it.
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: daniel.blueman@gmail.com
Link: http://lkml.kernel.org/r/20110518233157.744315519@sbsiddha-MOBL3.sc.intel.comSigned-off-by: Ingo Molnar <mingo@elte.hu>