Bring over the first part of the etherswitch framework - an MDIO bus and

Bring over the first part of the etherswitch framework - an MDIO bus and
MDIO/MII rendezvous proxy.

* Add an 'mdio' bus, which is the "IO" side of an MII bus (but by design
  can be anything which implements the underlying register access API.)
* Add 'miiproxy' and 'mdioproxy', which provides a rendezvous mechanism
  for MII busses to appear hanging off arbitrary busses (ie, that aren't
  necessarily a traditional looking MII bus.)

MII busses can now hang off anything that implements an mdiobus.

For the AR71xx SoC, there's one MDIO bus but two MII busses.  So to
properly support two or more real PHYs, this can be done:

  # arge0 MDIO bus - there's no arge1 MDIO bus for AR71xx
  hint.argemdio.0.at="nexus0"
  hint.argemdio.0.maddr=0x19000000
  hint.argemdio.0.msize=0x1000
  hint.argemdio.0.order=0

  # Create two mdioproxy instances
  hint.mdioproxy.0.at="mdio0"
  hint.mdioproxy.1.at="mdio0"

  # .. and with a follow-up patch
  hint.arge.0.mdio=mdioproxy0
  hint.arge.1.mdio=mdioproxy0

TODO:

* Do a sweep or two and add appropriate locking in mdio/mdioproxy/miiproxy.

Submitted by:	Stefan Bethke <stb@lassitu.de>
Reviewed by:	ray