PCI: acpiphp: Handle PCIe ports without native hotplug capability

Commit 0d52f54e (PCI / ACPI: Make acpiphp
ignore root bridges using PCIe native hotplug) added code that made the
acpiphp driver completely ignore PCIe root complexes for which the kernel
had been granted control of the native PCIe hotplug feature by the BIOS
through _OSC. Later commit 619a5182
"PCI hotplug: Always allow acpiphp to handle non-PCIe bridges" relaxed
the constraints to allow acpiphp driver handle non-PCIe bridges under
such a complex.  The constraint needs to be relaxed further to allow
acpiphp driver to handle PCIe ports without native PCIe hotplug capability.

Some MR-IOV switch chipsets, such PLX8696, support multiple virtual PCIe
switches and may migrate downstream ports among virtual switches.  To
migrate a downstream port from the source virtual switch to the target, the
port needs to be hot-removed from the source and hot-added into the target.
The pciehp driver can't be used here because there are no slots within the
virtual PCIe switch.  So acpiphp driver is used to support downstream port
migration.  A typical configuration is as below:

    [Root without native PCIe HP]
        [Upstream port of vswitch without native PCIe HP]
            [Downstream port of vswitch with native PCIe HP]
                [PCIe endpoint]

Here acpiphp driver will be used to handle root ports and upstream port
in the virtual switch, and pciehp driver will be used to handle downstream
ports in the virtual switch.

Signed-off-by: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>