Add comment to clarify that ignoring the MSB in the MAC is safe.
Add comment to clarify that ignoring the MSB in the MAC is safe.

It is not obvious that the MSB in the MAC of the multiplier can safely
be ignored, and it is tedious to derive this from solely studying the
architecture of the multiplier. This comment makes clear that it is
indeed safe and also gives some reasoning.

AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,AmbiML/ibex,lowRISC/ibex,AmbiML/ibex,lowRISC/ibex,lowRISC/ibex