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

arch/arm/boot/dts/aspeed/aspeed-bmc-ampere-mtmitchell.dts

@@ -813,7 +813,6 @@ bmc_ast2600_cpu: temperature-sensor@35 {
 };
 
 &adc0 {
-	ref_voltage = <2500>;
 	status = "okay";
 
 	pinctrl-names = "default";

arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-greatlakes.dts

@@ -211,7 +211,6 @@ &i2c13 {
 };
 
 &adc0 {
-	ref_voltage = <2500>;
 	status = "okay";
 	pinctrl-0 = <&pinctrl_adc0_default &pinctrl_adc1_default
 			&pinctrl_adc2_default &pinctrl_adc3_default
@@ -220,7 +219,6 @@ &pinctrl_adc4_default &pinctrl_adc5_default
 };
 
 &adc1 {
-	ref_voltage = <2500>;
 	status = "okay";
 	pinctrl-0 = <&pinctrl_adc8_default &pinctrl_adc10_default
 			&pinctrl_adc11_default &pinctrl_adc12_default

arch/arm/boot/dts/aspeed/aspeed-bmc-facebook-yosemite4.dts

@@ -603,7 +603,6 @@ i2c-mux@72 {
 };
 
 &adc0 {
-	ref_voltage = <2500>;
 	status = "okay";
 	pinctrl-0 = <&pinctrl_adc0_default &pinctrl_adc1_default
 			&pinctrl_adc2_default &pinctrl_adc3_default
@@ -612,7 +611,6 @@ &pinctrl_adc4_default &pinctrl_adc5_default
 };
 
 &adc1 {
-	ref_voltage = <2500>;
 	status = "okay";
 	pinctrl-0 = <&pinctrl_adc8_default &pinctrl_adc9_default>;
 };