powernv/pci: Move pnv_pci_dma_bus_setup() to pci-ioda.c

This is only used in pci-ioda.c so move it there and rename it to match.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200110070207.439-6-oohall@gmail.com