PCI: Update PCIEPORTBUS Kconfig help text

The Virtual Channel service has been removed and Downstream Port
Containment has been added, so update the symbol description to be
consistent with the current code.

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