ARM: stm32: Switch DHSOM to FMC2 EBI driver

Perform long overdue conversion of ad-hoc FMC2 EBI bus initialization
to upstream FMC2 EBI driver. No functional change.

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