PCI/ACPI: Notify acpi_pci_drivers when hot-plugging PCI root bridges

When hot-plugging PCI root bridge, acpi_pci_drivers' add()/remove()
methods should be invoked to notify registered drivers.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>