miiphy: Add function to retrieve MDIO bus list head

miiphy: Add function to retrieve MDIO bus list head

In upcoming freescale board LX2160AQDS, the MDIO bus is muxed.
i.e. same MDIO bus can be routed to eight different slots depending
on mux register settings.

To support this mdio mux behavior, we add each MDIO bus mux as a
separate MDIO bus.

Now, various phy devices can be attached to each of these slots(mux).
The information about these devices is passed to OS via device tree.

To do the fdt fixups related to MDIO bus, its necessary that MDIO bus
list is accessed.Therefore, add a function to retrieve the list head.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>