net: dsa: realtek: make interface drivers depend on OF

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>

drivers/net/dsa/realtek/Kconfig

@@ -12,6 +12,7 @@ menuconfig NET_DSA_REALTEK
 config NET_DSA_REALTEK_MDIO
 	tristate "Realtek MDIO connected switch driver"
 	depends on NET_DSA_REALTEK
+	depends on OF
 	help
 	  Select to enable support for registering switches configured
 	  through MDIO.
@@ -19,6 +20,7 @@ config NET_DSA_REALTEK_MDIO
 config NET_DSA_REALTEK_SMI
 	tristate "Realtek SMI connected switch driver"
 	depends on NET_DSA_REALTEK
+	depends on OF
 	help
 	  Select to enable support for registering switches connected
 	  through SMI.