net: mdio-octeon: Modify driver to work on both ThunderX and Octeon

This patch modifies the mdio-octeon driver to work on both ThunderX and
Octeon SoCs from Cavium Inc.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>