netdev/of/phy: Add MDIO bus multiplexer support.

This patch adds a somewhat generic framework for MDIO bus
multiplexers.  It is modeled on the I2C multiplexer.

The multiplexer is needed if there are multiple PHYs with the same
address connected to the same MDIO bus adepter, or if there is
insufficient electrical drive capability for all the connected PHY
devices.

Conceptually it could look something like this:

                   ------------------
                   | Control Signal |
                   --------+---------
                           |
 ---------------   --------+------
 | MDIO MASTER |---| Multiplexer |
 ---------------   --+-------+----
                     |       |
                     C       C
                     h       h
                     i       i
                     l       l
                     d       d
                     |       |
     ---------       A       B   ---------
     |       |       |       |   |       |
     | PHY@1 +-------+       +---+ PHY@1 |
     |   ...