ARM: dts: aspeed: drop unused ref_voltage ADC property

Aspeed ADC "ref_voltage" property is neither documented nor used.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240405064624.18997-5-krzysztof.kozlowski@linaro.org


Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>