dt-bindings: thermal: mediatek: add compatible string for MT7986 and MT7981 SoC

Document compatible string 'mediatek,mt7986-thermal' for V3 thermal
unit found in MT7986 SoCs.
'mediatek,mt7981-thermal' is also added as it is identical with the
thermal unit of MT7986.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>