boards: arm: nrf5340_audio_dk_nrf5340: add board id description

nRF5340 Audio DK embeds a voltage divider to identify the "board ID".
Use the existing 'voltage-divider' binding to describe it. Also
configure the ADC channel 0 to read the voltage divider value.
Configuration from
https://github.com/nrfconnect/sdk-nrf/blob/main/applications/


nrf5340_audio/src/utils/board_version.c
has been taken.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>