boards stm32h747i_disco: Add missing mdio node

Akin to the changes in
ab29ee5e0b3e07afdca308255487703d940ced32
Add missing mdio node for
stm32h747i_disco/stm32h747i_disco_stm32h747xx_m7

Without this, samples/net/sockets/echo_server
fails to build in this board.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>