powerpc/pci: add dma_set_mask to pci_controller_ops

Some systems only need to deal with DMA masks for PCI devices.
For these systems, we can avoid the need for a platform hook and
instead use a pci controller based hook.

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