Update lopper demo to work on current code base

[wmamills]

The lopper demo in uses a commit from Dec of 2022 (branch systemdt-linaro-demo) and should be updated to work from master.
@tnmysh had suggestions for demo improvement as well that will effect the demo script (here and the demo README in the lopper repo.)

[tnmysh]

This issue is addressed in lopper PR: devicetree-org/lopper#404

[nathalie-ckc]

Since the Lopper PR is merged, the next step is for Tanmay to update the readme in the OpenAMP docs repo.