dt-bindings: thermal: Drop 'trips' node as required

It is possible to have thermal zones which don't have any trip points.
These zones in effect simply represent a temperature sensor without any
action associated with it. While the schema has always required a
'trips' node, users have existed for a long time without it. Update the
schema to match reality.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240709150154.3272825-1-robh@kernel.org


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>