[prim] Minor work-around for xcelium
[prim] Minor work-around for xcelium

- As @matute noted, xcelium incorrectly does not allow empty parameters.
- Workaround for now by assigning a default value

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

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