ARM: stm32: Fix SoM and board coding strap GPIO handling on DHSOM

The variables retaining the strap values have to be initialized, always,
make it so. Moreover, free the requested GPIO list at the end to avoid
wasting memory.

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>