dt-bindings: thermal: Make trips node optional

Even though the previous binding made it a required child node, the
implementation in Linux never made it mandatory and just ignored thermal
zones without trip points.

This was even effectively encouraged, since the thermal core wouldn't
allow a thermal sensor to probe without a thermal zone.

In the case where you had a thermal device that had multiple sensors but
with enough knowledge to provide trip points for only a few of them,
this meant that the only way to make that driver probe was to provide a
thermal zone without the trips node required by the binding.

This obviously led to a fair number of device trees doing exactly that,
making the initial binding requirement ineffective.

Let's make it clear by dropping that requirement.

Cc: Amit Kucheria <amitk@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210721140424.725744-34-maxime@cerno.tech

Documentation/devicetree/bindings/thermal/thermal-zones.yaml

@@ -215,7 +215,7 @@ patternProperties:
       - polling-delay
       - polling-delay-passive
       - thermal-sensors
-      - trips
+
     additionalProperties: false
 
 additionalProperties: false