powerpc/macio: Ensure all dma routines get copied over

Also add a comment to dev_archdata, indicating that changes there need
to be verified against the driver code.

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