powerpc/pci: Add shutdown hook to pci_controller_ops

Currently pnv_pci_shutdown() calls the PHB shutdown code for all PHBs in the
system.  It dereferences the private_data assuming it's a powernv PHB, which
won't be the case when we have different PHB in the systems (like when we add
vPHBs for CXL).

This moves the shutdown hook to the pci_controller_ops and fixes the call site
to use that instead.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>