dt-bindings: iio: adc: amlogic,meson-saradc: add optional power-domains

On newer SoCs, the SAR ADC hardware can require a power-domain to operate,
add it as optional.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240605-topic-amlogic-upstream-bindings-fixes-power-domains-sardac-v1-1-40a8de6baa59@linaro.org


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