pci, ACPI, iommu: Enhance pci_root to support DMAR device hotplug

Finally enhance pci_root driver to support DMAR device hotplug when
hot-plugging PCI host bridges.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: Yijing Wang <wangyijing@huawei.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>