netdev: bfin_mac: add Blackfin MII bus to platform bus to allow DSA access

When we register the MII bus to the platfrom bus, the Distributed Switch
Architecture can hook in transparently.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>