net: Rework mpc5200 fec driver to use of_mdio infrastructure.

The patch reworks the MPC5200 Fast Ethernet Controller (FEC) driver to
use the of_mdio infrastructure for registering PHY devices from data out
openfirmware device tree, and eliminates the assumption that the PHY
for the FEC is always attached to the FEC's own MDIO bus.  With this
patch, the FEC can use a PHY attached to any MDIO bus if it is described
in the device tree.

Tested on Freescale Lite5200b eval board

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>