x86/PCI: add pci_mmconfig_insert()/delete() for PCI root bridge hotplug

Introduce pci_mmconfig_insert()/pci_mmconfig_delete(), which will be used
to update MMCONFIG information when supporting PCI root bridge hotplug.

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>