[TF-M] TF-M copyright year update

Hi,

This is a continue of discussion started on today's forum on the value of copyright year update on every change. I proposed to let such update being optional and do not track it in review. There was no objection during forum but an alternative proposal to adopt the Linux Foundation guidancehttps://www.linuxfoundation.org/blog/blog/copyright-notices-in-open-source-software-projects, where all authors and contributors are listed in a separate while source and documentation are free from it. For example MbedTLS doing that way, although it has side effects:

* Need to modify all files in 6 repositories. * Gather permission from all copyright holders.

Looking for more opinions on the topic.

Thanks, Anton