Implement voltage sense at the header board

[nkrackow]

The AI-ARTIQ laser module header board has an additional voltage sensing stage that goes to another MCU ADC pin. That is a better point for voltage sensing due to sinara-hw/Laser_Module#9 and should be implemented.

[jordens]

This monitoring channel was done out of https://github.com/quartiq/ai-artiq/issues/29#issuecomment-1094380218