dt-bindings: thermal: mediatek: Add fallback compatible string for MT7981 and MT8516

dt-bindings: thermal: mediatek: Add fallback compatible string for MT7981 and MT8516

The ‘mediatek,mt7981-thermal’ and ‘mediatek,mt8516-thermal’ strings
aren't definied in the driver. Both should have fallback compatible
strings. This commit fixes this issue.

Fixes: 788494ba0999 ("dt-bindings: thermal: convert Mediatek Thermal to the json-schema")
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20250712195904.6988-2-olek2@wp.pl
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>