PCI: Add hotplug_slot_ops.reset_slot()

This optional callback allows hotplug controllers to perform slot
specific resets.  These may be necessary in cases where a normal
secondary bus reset can interact with controller logic and expose
spurious hotplugs.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>