powerpc: dart_iommu: optionally populate controller_ops on init

If a pci_controller_ops struct is provided to iommu_init_early_dart,
populate that with the DMA setup ops, rather than ppc_md. If NULL is
provided, populate ppc_md as before.

This also patches the call sites for Maple and Power Mac to pass
NULL, so existing behaviour is preserved.

The benefit of making this optional is that it means we don't have
to change dart, Maple and Power Mac over to the controller_ops
system in one fell swoop.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>