x86, ioapic: Define IO_APIC_DEFAULT_PHYS_BASE constant

We already have APIC_DEFAULT_PHYS_BASE so just to be
consistent.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
LKML-Reference: <20090824175550.927946757@openvz.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>