net: bcmgenet: add MDIO routines

This patch adds support for configuring the port multiplexer hardware
which resides in front of the GENET Ethernet MAC controller. This allows
us to support:

- internal PHYs (using drivers/net/phy/bcm7xxx.c)
- MoCA PHYs which are an entirely separate hardware block not covered
  here
- external PHYs and switches

Note that MoCA and switches are currently supported using the emulated
"fixed PHY" driver.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>