Use dcr_host_t.base in ibm_emac_mal

This requires us to do a sort-of fake dcr_map(), so that base is set
properly. This will be fixed/removed when the device-tree-aware emac driver
is merged.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jeff Garzik <jeff@garzik.org>