powerpc/pasemi: Clean up pasemi iommu table initializations

No need for empty helpers with iommu off, the ppc_md hooks are optional.

The direct_dma_ops are the default pci_dma_ops, so no need to set in the
them iommu off case.

No need to set the device tree device_node pci node iommu pointer, its
only used for dlpar remove.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>