mac_dbdma: remove DBDMA_init() function

Instead we can now instantiate the MAC_DBDMA object directly within the
macio device. We also add the DBDMA device as a child property so that
it is possible to retrieve later.

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