x86/PCI: update MMCONFIG information when hot-plugging PCI host bridges

This patch enhances x86 arch-specific code to update MMCONFIG information
when PCI host bridge hotplug event happens.

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