powerpc/82xx: Rework Embedded Planet ep8248e platform to use of_mdio

This patch modifies the bitbanged MDIO driver in the ep8248e platform
code to use the common of_mdio infrastructure.

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>