dt-bindings: iio: temperature: ltc2983: document power supply

Add a property for the VDD power supply regulator.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240404-ltc2983-misc-improv-v5-1-c1f58057fcea@analog.com


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