dt-bindings: i2c: i2c-mt65xx: add MediaTek MT7981 SoC

Add compatible string for the I2C controllers found in the MediaTek
MT7981 SoC.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>