powerpc/pci: Hotplug driver bridge support

There is an issue with the hotplug operation when it's done on the
bridge/switch slot. The bridge-port and devices behind the bridge, which
become offline by hot-unplug operation, don't get hot-plugged/enabled
by doing hot-plug operation on that slot. Only the first port of the
bridge gets enabled and the remaining port/devices remain unplugged. The
hot plug/unplug operation is done by the hotplug driver (drivers/pci/
hotplug/pnv_php.c).

This behavior is due to missing code for the switch/bridge. The existing
driver depends on pci_hp_add_devices() function for device enablement.
This function calls pci_scan_slot() on only one device-node/port of the
bridge, not on all the siblings' device-node/port.

The missing code needs to be added which will find all the sibling
device-nodes/bridge-ports and will run explicit pci_scan_slot()
on those. A new function has been added for this purpose
which is invoked from pci_hp_add_devices(). This new function
traverse_siblings_and_scan_slot() gets all the sibling bridge-ports by
traversal and explicitly invokes pci_scan_slot() on them.

Tested-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Signed-off-by: Krishna Kumar <krishnak@linux.ibm.com>
[mpe: Move the code into pci-hotplug.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240701074513.94873-3-krishnak@linux.ibm.com