[otbn,dv] Adjust assertions in otbn_rnd_if to handle ignored values
[otbn,dv] Adjust assertions in otbn_rnd_if to handle ignored values

The changes made in the RTL in 8e97525 and the documentation in
46e134f allow a situation where data comes in from the EDN and is
ignored.

This commit tweaks the FSM diagram and associated assertions
accordingly. I've also made signals for the transition labels to try
to decouple the assertions and RTL signal names a little.

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

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