[Xcelium support] Remove void from DPI definition

Created by: CoralieAllioux

Those minor fixes are suggested in the context of supporting Cadence Xcelium simulator for cva6 verification. Xcelium does not allow the DPI function to be to return a void value.

This means in CVA6 repository, the reference to this function must be updated! If a positive feedback is given, I should created a PR should on cva6 repository and update the core-v-verif hash submodule.

This has been check with VCS 2021.09: the behavior is the same than previous on our side.