dt-bindings: thermal: imx8mm: reference thermal-sensor schema

Device is a thermal sensor and it requires '#thermal-sensor-cells', so
reference the thermal-sensor.yaml to simplify it and bring the
common definition of '#thermal-sensor-cells' property.

Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>

Documentation/devicetree/bindings/thermal/imx8mm-thermal.yaml

···
   for i.MX8MM which has ONLY 1 sensor, v2 is for i.MX8MP which has
   2 sensors.
 
+$ref: thermal-sensor.yaml#
+
 properties:
   compatible:
     oneOf:
···
   - compatible
   - reg
   - clocks
-  - '#thermal-sensor-cells'
 
-additionalProperties: false
+unevaluatedProperties: false
 
 examples:
   - |