Add a 'thermal-zones' node for stih418.

A thermal-zone needs three components:
  - thermal sensors, described in an earlier commit[1]
  - cooling devices, specified for each CPU
  - a thermal zone, describing the overall behavior.

The thermal zone needs references to both CPUs and thermal sensors,
which phandle are also added. The thermal management will then be
achieved on CPUs using the cpufreq framework.

[1] https://lore.kernel.org/lkml/20240320-thermal-v3-2-700296694c4a@gmail.com/Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>