ARM: stm32: Use __section(".data") with dot in the section name on DHSOM

The correct specifier of the section is ".data" and not "data",
use the former to place the variables in ".data" section.

Fixes: 731fd50e ("ARM: stm32: Implement board coding on AV96")
Fixes: 92ca0f74 ("ARM: dts: stm32: Synchronize DDR setttings on DH SoMs")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>