net: thunderx: Select CONFIG_MDIO_OCTEON for ThunderX NIC

The CONFIG_MDIO_OCTEON is required so that the ThunderX NIC driver can
talk to the PHY drivers.

Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>