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: 379d7ac7ca31 ("phy: mdio-thunder: Add driver for Cavium Thunder SoC MDIO buses.")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Bartosz Golaszewski <[e-mail]>
Cc: Andrew Lunn <[e-mail]>
Cc: Heiner Kallweit <[e-mail]>
Cc: [e-mail]
Cc: David Daney <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/Kconfig

···
 	depends on 64BIT
 	depends on PCI
 	select MDIO_CAVIUM
+	select MDIO_DEVRES
 	help
 	  This driver supports the MDIO interfaces found on Cavium
 	  ThunderX SoCs when the MDIO bus device appears as a PCI