ARM: stm32: Add version variable to DHSOM

Enable insertion of version variable into U-Boot environment on DHSOM,
to make it possible to check U-Boot version e.g. in U-Boot scripts.

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