ARM: dts: stm32: Add KS8851-16MLL ethernet on FMC2

Add DT entries, Kconfig entries and board-specific entries to configure
FMC2 bus and make KS8851-16MLL on that bus accessible to U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>