ppc/mac: More rework of the DBDMA emulation

ppc/mac: More rework of the DBDMA emulation

This completely reworks the handling of the control register
according to my understanding of the HW and the spec.

It should (hopefully ... still testing) fix a number of issues
most notably cases of MacOS hanging.

Also update dbdma_unassigned_rw() and dbdma_unassigned_flush() to
have the expected behaviour now that flush is handled slightly
differently.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>