dt-bindings: imx-thermal: Add imx6sll and imx6ul compatible

Currently the dtbs_check for imx6 generates warnings like this:

['fsl,imx6sll-tempmon', 'fsl,imx6sx-tempmon'] is too long

So add them to the devicetree binding.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230410205803.45853-4-stefan.wahren@i2se.com

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

@@ -12,10 +12,16 @@ maintainers:
 
 properties:
   compatible:
-    enum:
-      - fsl,imx6q-tempmon
-      - fsl,imx6sx-tempmon
-      - fsl,imx7d-tempmon
+    oneOf:
+      - enum:
+          - fsl,imx6q-tempmon
+          - fsl,imx6sx-tempmon
+          - fsl,imx7d-tempmon
+      - items:
+          - enum:
+              - fsl,imx6sll-tempmon
+              - fsl,imx6ul-tempmon
+          - const: fsl,imx6sx-tempmon
 
   interrupts:
     description: |