Restore support for the 5706C bce(4) phy that was broken during the
addition of SerDes support. According to the docs, the 5706C and 5708C
phys are supposed to use the same MII model that is separate from the
SerDes parts, but the 5706C actually uses the MII model of the SerDes
parts. To fix this, readd the old 5706C entry to miidevs and add a
special check in brgphy_probe() for phys that match the 5706C ID. If
the phy is supported by the gentbi(4) driver, then it's a SerDes phy, so
we fail the probe and let gentbi(4) grab it. Otherwise, it's a 5706C phy,
so we let brgphy(4) grab it.

In coordination with: dwhite