PCI: Include PCI-to-PCIe bridges as "Downstream Ports"

A PCI/PCI-X to PCI Express bridge, sometimes referred to as a "reverse
bridge", is a bridge with conventional PCI or PCI-X on its primary side and
a PCI Express Port on its secondary (downstream) side.

That PCIe Port is a Downstream Port and could be connected to a slot, just
like a Root Port or a Switch Downstream Port.  Make pcie_downstream_port()
return true for them, so we can access the Slot registers in the PCIe
capability.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>