ARM: stm32mp: skip ft_system_setup when the soc node is absent

ARM: stm32mp: skip ft_system_setup when the soc node is absent

The function ft_system_setup shouldn't return an error when the
/soc node is absent in the provided device tree but just skip the
updates.

This patch solves an issue when the U-Boot pytest is executed on board.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>