[TF-M] Process for deprecating and removing platforms

Hi Everyone,

The document in tf.org defines the different "Platform Support Life Cycle":

https://developer.trustedfirmware.org/w/collaboration/project-maintenance-pr...

Although a platform can be implicitly in deprecated state due to lack of support or maintenance, a process to explicitly declare a platform as deprecated with the intention to remove it in future is needed as well. So I have attempted to define a process as described here: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/4962/1/docs/c...

Comments welcome.

Best Regards Soby Mathew