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>