Add a set of constants describing the ways a MAC and PHY can be connected.

While the initial need for this is to help support phy drivers which are
configured with FDT data, there is nothing devicetree-specific about the
concept or the names, so they are available for use even on non-FDT systems.

The initial list of connection types comes from the current devicetree
bindings documentation, but values not documented there can be added to
the list in the future as needed, the values could be sorted into a
different order without perturbing FDT code, etc. The only invariant
is that MII_CONTYPE_UNKNOWN should be first (so it has a value of zero,
so that a con-type variable in a softc, for example, is initialized to
MII_CONTYPE_UNKNOWN by default).