drivers: adc: nrfx_saadc: Add validation of channel configuration

On nrf54h20 there are additional analog pins (AIN8+). When differential
mode is used they must not be mixed with AIN0-AIN7. Add build time
validation which detects if configuration is invalid.

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