ARM: dts: sti: add the thermal sensor node within stih418

The STiH418 embedded the same sensor as the STiH410.
This commit adds the corresponding node, relying on the st_thermal
driver.

Signed-off-by: Alain Volmat <avolmat@me.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>