net: phy: add a Kconfig option for mdio_devres

If phylib is built as a module and CONFIG_MDIO_DEVICE is 'y', the
mdio_device and mdio_bus code will be in the phylib module, not in the
kernel image. Meanwhile we build mdio_devres depending on the
CONFIG_MDIO_DEVICE symbol, so if it's 'y', it will go into the kernel
and we'll hit the following linker error:

   ld: drivers/net/phy/mdio_devres.o: in function `devm_mdiobus_alloc_size':
>> drivers/net/phy/mdio_devres.c:38: undefined reference to `mdiobus_alloc_size'
   ld: drivers/net/phy/mdio_devres.o: in function `devm_mdiobus_free':
>> drivers/net/phy/mdio_devres.c:16: undefined reference to `mdiobus_free'
   ld: drivers/net/phy/mdio_devres.o: in function `__devm_mdiobus_register':
>> drivers/net/phy/mdio_devres.c:87: undefined reference to `__mdiobus_register'
   ld: drivers/net/phy/mdio_devres.o: in function `devm_mdiobus_unregister':
>> drivers/net/phy/mdio_devres.c:53: undefined reference to `mdiobus_unregister'
   ld: drivers/net/phy/mdio_devres.o: in function `devm_of_mdiobus_register':
>> drivers/net/phy/mdio_devres.c:120: undefined reference to `of_mdiobus_register'

Add a hidden Kconfig option for MDIO_DEVRES which will be currently
selected by CONFIG_PHYLIB as there are no non-phylib users of these
helpers.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: ac3a68d5 ("net: phy: don't abuse devres in devm_mdiobus_register()")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>