PCI: pnv_php: Fix surprise plug detection and recovery

The existing PowerNV hotplug code did not handle surprise plug events
correctly, leading to a complete failure of the hotplug system after device
removal and a required reboot to detect new devices.

This comes down to two issues:

 1) When a device is surprise removed, often the bridge upstream
    port will cause a PE freeze on the PHB.  If this freeze is not
    cleared, the MSI interrupts from the bridge hotplug notification
    logic will not be received by the kernel, stalling all plug events
    on all slots associated with the PE.

 2) When a device is removed from a slot, regardless of surprise or
    programmatic removal, the associated PHB/PE ls left frozen.
    If this freeze is not cleared via a fundamental reset, skiboot
    is unable to clear the freeze and cannot retrain / rescan the
    slot.  This also requires a reboot to clear the freeze and redetect
    the device in the slot.

Issue the appropriate unfreeze and rescan commands on hotplug events,
and don't oops on hotplug if pci_bus_to_OF_node() returns NULL.

Signed-off-by: Timothy Pearson <[email protected]>
[bhelgaas: tidy comments]
Signed-off-by: Bjorn Helgaas <[email protected]>
Signed-off-by: Madhavan Srinivasan <[email protected]>
Link: https://patch.msgid.link/171044224.1359864.1752615546988.JavaMail.zimbra@raptorengineeringinc.com

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