samples: drivers: adc: adc_sequence: Fix nrf54h20dk configuration

AIN9 source needs special handling (all from AIN8-AIN14). Conversion
result needs to be scaled up by 3.6 additionally to gain and reference
voltage. In order to achieve correct value in the sample vref-mv is
used as reference voltage instead of predefined internal reference
source.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

samples/drivers/adc/adc_sequence/boards/nrf54h20dk_nrf54h20_cpuapp.overlay

@@ -42,9 +42,9 @@
 	channel@2 {
 		reg = <2>;
 		zephyr,gain = "ADC_GAIN_1_2";
-		zephyr,reference = "ADC_REF_INTERNAL";
 		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
 		zephyr,input-positive = <NRF_SAADC_AIN9>; /* P9.01 */
+		zephyr,vref-mv = <3686>; /* 3.6V * 1024 */
 		zephyr,resolution = <12>;
 		zephyr,oversampling = <8>;
 	};