[POWERPC] Use archdata.dma_data in dma_direct_ops and add the offset

Now that all platforms using dma_direct_offset setup the
archdata.dma_data correctly, we can change the dma_direct_ops to
retrieve the offset from the dma_data, rather than directly from the
global.

While we're here, change the way the offset is used - instead of
or'ing it into the value, add it.  This should have no effect on
current implementations where the offset is far larger than memory,
however in future we may want to use smaller offsets.

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>