mdio: fix mdio-thunder.c dependency & build error

Fix build error by selecting MDIO_DEVRES for MDIO_THUNDER.
Fixes this build error:

ld: drivers/net/phy/mdio-thunder.o: in function `thunder_mdiobus_pci_probe':
drivers/net/phy/mdio-thunder.c:78: undefined reference to `devm_mdiobus_alloc_size'

Fixes: 379d7ac7 ("phy: mdio-thunder: Add driver for Cavium Thunder SoC MDIO buses.")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: netdev@vger.kernel.org
Cc: David Daney <david.daney@cavium.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>