netdev/phy: add MDIO bus multiplexer driven by a memory-mapped device

Add support for an MDIO bus multiplexer controlled by a simple memory-mapped
device, like an FPGA.  The device must be memory-mapped and contain only
8-bit registers (which keeps things simple).

Tested on a Freescale P5020DS board which uses the "PIXIS" FPGA attached
to the localbus.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>