ARM: dts: aspeed: wspoon: Enable iio-hwmon battery

The BMC can read the RTC battery voltage via ADC
channel 12.
Signed-off-by: NMatt Spinler <spinler@linux.vnet.ibm.com>
Reviewed-by: NLei YU <mine260309@gmail.com>
Signed-off-by: NJoel Stanley <joel@jms.id.au>

arch/arm/boot/dts/aspeed-bmc-opp-witherspoon.dts

@@ -56,6 +56,11 @@
 		};
 	};
 
+	iio-hwmon-battery {
+		compatible = "iio-hwmon";
+		io-channels = <&adc 12>;
+	};
+
 	gpio-keys-polled {
 		compatible = "gpio-keys-polled";
 		#address-cells = <1>;
@@ -583,3 +588,7 @@
 &ibt {
 	status = "okay";
 };
+
+&adc {
+	status = "okay";
+};