802.3 doens't specify ANAR or ANLPAR bits for GMII. Need to
use PHY-specific registers for this, for now, which means we
need specific drivers for the Gigabit-capable PHYs (I think the
most common is the BCM5400).
mapping an OUI to the MII id registers.
Doing this in the MII_OUI() macro would be too complex, so put it into
a helper function and move the MII id register interpretation stuff
into miivar.h.
