[mii] Separate concepts of MII interface and MII device

[mii] Separate concepts of MII interface and MII device

We currently have no generic concept of a PHY address, since all
existing implementations simply hardcode the PHY address within the
MII access methods.

A bit-bashing MII interface will need to be provided with an explicit
PHY address in order to generate the correct waveform.  Allow for this
by separating out the concept of a MII device (i.e. a specific PHY
address attached to a particular MII interface).

Signed-off-by: Michael Brown <mcb30@ipxe.org>