dt-bindings: iio: humidity: hdc3020: add reset-gpios

The HDC3020 provides an active low reset signal that is still not
described in the bindings.

Add reset-gpios to the bindings and the example.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240303-hdc3020-pm-v3-2-48bc02b5241b@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>