The kernel test robot reported build warnings with a randconfig that
built realtek-{smi,mdio} without CONFIG_OF set. Since both interface
drivers are using OF and will not probe without, add the corresponding
dependency to Kconfig.

Link: https://lore.kernel.org/all/202203231233.Xx73Y40o-lkp@intel.com/
Link: https://lore.kernel.org/all/202203231439.ycl0jg50-lkp@intel.com/
Fixes: aac94001 ("net: dsa: realtek: add new mdio interface for drivers")
Fixes: 765c39a4 ("net: dsa: realtek: convert subdrivers into modules")
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Link: https://lore.kernel.org/r/20220323124225.91763-1-alvin@pqrs.dkSigned-off-by: Jakub Kicinski <kuba@kernel.org>