[otbn,dv] Tweak how we randomise flip_bits in otbn_common_vseq
[otbn,dv] Tweak how we randomise flip_bits in otbn_common_vseq

Xcelium doesn't allow randomising an entry in an arry ("The argument
to class or scope randomize must be a simple identifier of integral
type.").

It turns out that we can shuffle things around a bit to simplify how
everything works, so I've done that too.

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

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