[otbn] unify montmul routines in modexp
[otbn] unify montmul routines in modexp

This unfiies the three Montgomery multiplication
subroutine montmul_sqr, montmul_exp and montmul
in a single one.

This is related to #3776.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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