powerpc/powernv: move dma_get_required_mask from pnv_phb to pci_controller_ops

powerpc/powernv: move dma_get_required_mask from pnv_phb to pci_controller_ops

Simplify the dma_get_required_mask call chain by moving it from pnv_phb to
pci_controller_ops, similar to commit 763d2d8df1ee ("powerpc/powernv:
Move dma_set_mask from pnv_phb to pci_controller_ops").

Previous call chain:

  0) call dma_get_required_mask() (kernel/dma.c)
  1) call ppc_md.dma_get_required_mask, if it exists. On powernv, that
     points to pnv_dma_get_required_mask() (platforms/powernv/setup.c)
  2) device is PCI, therefore call pnv_pci_dma_get_required_mask()
     (platforms/powernv/pci.c)
  3) call phb->dma_get_required_mask if it exists
  4) it only exists in the ioda case, where it points to
       pnv_pci_ioda_dma_get_required_mask() (platforms/powernv/pci-ioda.c)

New call chain:

  0) call dma_get_required_mask() (kernel/dma.c)
  1) device is PCI, therefore call pci_controller_ops.dma_get_required_mask
     if it exists
  2) in the ioda case, that points to pnv_pci_ioda_dma_get_required_mask()
     (platforms/powernv/pci-ioda.c)

In the p5ioc2 case, the call chain remains the same -
dma_get_required_mask() does not find either a ppc_md call or
pci_controller_ops call, so it calls __dma_get_required_mask().

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>