net: mdio: octeon: Select MDIO_DEVRES

This driver makes use of devm_mdiobus_alloc_size. To ensure this is
available select MDIO_DEVRES which provides it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>