dt-bindings: iio: adc: Add support for MT7981

The temperature sensor in the MT7981 is same as in the MT7986.
Add compatible string for mt7981.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250708220405.1072393-2-olek2@wp.pl


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>