ARM: stm32: enable support for smsc phy on stm32f746-disco board

This patch enables support for the smsc phy on the
stm32f746-disco board.

Signed-off-by: Michael Kurz <michi.kurz@gmail.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>

Series-changes 3:
- Add Acked-by tag to 'enable support for smsc phy on...'