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>