[otbn,dv] Initialise some variables in otbn_core_model
[otbn,dv] Initialise some variables in otbn_core_model

I'm a bit surprised that these came up as 0, not X. But we should
probably not rely on that! :-)

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

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