[TF-A] Updates to the Change Log process

Hi,

We are intending to move the updates to the change log from completely release based to more rolling updates. I have created a new change log file in the docs directory (change-log-upcoming.rst) for changes that need to be included for the upcoming release: https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/3081. The intention is for individual developers to update the document with the relevant patch as the code change is being made. The usual change log document (docs/change-log.rst) will be updated at release time to include all changes for each release.

Thanks,

Lauren