ARM: stm32: activate OF_LIVE for DHSOM

Activate the live DT with CONFIG_OF_LIVE to reduce the DT parsing
time.

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