PCI/VMD: Configure MPS settings before adding devices

In order to provide the most performance and/or compatible settings,
ensure VMD root buses observe the pcie bus tuning settings by
configuring those settings prior to adding the devices to the pcie tree.

This patch open-codes pci_rescan_bus() and configures the buses prior to
adding devices and attaching drivers.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>