drivers: ethernet: stm32: Use MDIO API only if enabled by DTS

Not all STM32 series support Zephyr MDIO API yet, while the API is enabled
by default.
To preserve compatibility, put MDIO API related code under the condition
of "st,stm32-mdio" compatible enablement.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>