On some platforms, root port has neither MSI/MSI-X nor INTx interrupt
generated in RC mode. In this case, we have to use other interrupt, e.g.,
system shared interrupt, for port service IRQ to have AER, Hot-plug, etc.,
services work.
Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>