thermal: rcar-thermal: update thermal zone only when temperature changes

Avoid updating the thermal zone in case an IRQ was triggered but the
temperature didn't effectively change.
Note this is not a driver issue.
Below is a captured debug trace illustrating the purpose of this patch:
out of 8 thermal zone updates, only 2 are actually necessary.

[   41.120000] rcar_thermal_work(): cctemp=25000
[   41.120000] rcar_thermal_work(): nctemp=30000
[   41.120000] rcar_thermal_work(): temp is now 30000C, update thermal zone
[   58.990000] rcar_thermal_work(): cctemp=30000
[   58.990000] rcar_thermal_work(): nctemp=30000
[   58.990000] rcar_thermal_work(): same temp, do not update thermal zone
[   59.290000] rcar_thermal_work(): cctemp=30000
[   59.290000] rcar_thermal_work(): nctemp=30000
[   59.290000] rcar_thermal_work(): same temp, do not update thermal zone
[   59.590000] rcar_thermal_work(): cctemp=30000
[   59.590000] rcar_thermal_work(): nctemp=30000
[   59.590000] rcar_thermal_work(): same temp, do not update thermal zone
[   59.890000] rcar_thermal_work(): cctemp=30000
[   59.890000] rcar_thermal_work(): nctemp=30000
[   59.890000] rcar_thermal_work(): same temp, do not update thermal zone
[   60.190000] rcar_thermal_work(): cctemp=30000
[   60.190000] rcar_thermal_work(): nctemp=30000
[   60.190000] rcar_thermal_work(): same temp, do not update thermal zone
[   60.490000] rcar_thermal_work(): cctemp=30000
[   60.490000] rcar_thermal_work(): nctemp=30000
[   60.490000] rcar_thermal_work(): same temp, do not update thermal zone
[   60.790000] rcar_thermal_work(): cctemp=30000
[   60.790000] rcar_thermal_work(): nctemp=35000
[   60.790000] rcar_thermal_work(): temp is now 35000C, update thermal zone

I suspect this may be due to sensor sampling accuracy / fluctuation,
but no formal proof.

Signed-off-by: Patrick Titiano <ptitiano@baylibre.com>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>