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