net: cavium: For Kconfig THUNDER_NIC_BGX, select MDIO_THUNDER.

Previously we selected MDIO_OCTEON, which after creating the Thunder
specific MDIO bus driver is much less useful.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>