PCI: Add MCFG quirks for X-Gene host controller

PCIe controllers in X-Gene SoCs are not ECAM compliant: software needs to
configure additional controller's register to address device at
bus:dev:function.

Add a quirk to discover controller MMIO register space and configure
controller registers to select and address the target secondary device.

The quirk will only be applied for X-Gene PCIe MCFG table with
OEM revison 1, 2, 3 or 4 (PCIe controller v1 and v2 on X-Gene SoCs).

Tested-by: Jon Masters <jcm@redhat.com>
Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>