[entropy_src/dv] Bug fix for Xcelium
[entropy_src/dv] Bug fix for Xcelium

Xcelium does not support the logical operation: !&some_vector.

Adds some comments to clarify the rationale for the overflow-protected
counters.

Signed-off-by: Martin Lueker-Boden <7e82c17fafb3b7b91b3c3396a2f21a060e5a6c3d@wdc.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan