Documentation: tpm: Add documentation for the CRB FF-A interface

Add documentation providing details of how the CRB driver interacts with
ARM FF-A.

[jarkko: Fine-tuned the commit message.]

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>