powerpc/powernv: Introduce pnv_pci_get_slot_id()

This introduces pnv_pci_get_slot_id() to get the hotpluggable PCI
slot ID from the corresponding device node. It will be used by
hotplug driver.

Requested-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>