[otbn,dv] Define coverage for pairwise instruction mnemonics
[otbn,dv] Define coverage for pairwise instruction mnemonics

This says that we want to see each instruction (mnemonic) followed by
every other instruction. The only exception is that ECALL isn't
followed by anything else.

A quick run of 10 seeds for otbn-single hits roughly 20% of the cross.
Hopefully a full overnight run will do a bit better.

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

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