PCI/ACPI: Assign unassigned resource for hot-added root bus

After we get hot-added IOAPIC registered.
pci_enable_bridges() will try to enable IOAPIC IRQ for PCI bridges.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>