board: stm32f746-disco: Get MII/RMII phy_mode from DT

This is needed to make ethernet work on stm32f746-eval which
uses MII mode.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>