[otbn,dv] Updates to planned fcov for BN instruction set
[otbn,dv] Updates to planned fcov for BN instruction set

These address on follow-up comments to PR #6491 (which came in after
the PR was merged).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

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