It adds a new board operation for auto-clear any offset. If a board
does not support this feature in its hardware or driver, then it can
use the software implementation. The software implementation does an
acquisition on disconnected channel and compute the signal average;
we assume that this average is a constant noise, then we apply this
value to the DAC to clear it on the ADC input.
Signed-off-by: Federico Vaga <federico.vaga@cern.ch>