[POWERPC] Set archdata.dma_data for direct DMA in cell_dma_dev_setup()

Store the direct_dma_offset in each device's dma_data in the case
where we're using the direct DMA ops.

We need to make sure we setup the ppc_md.pci_dma_dev_setup() callback
if we're using a non-zero offset.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>