Extend OP-TEE concept for virtualization and advanced topologies

The current OP-TEE concept document covers the basic usage of OP-TEE however
product groups will be looking to go far beyond that level. Update the concept
document to more fully cover the use virualization with OP-TEE and show how we
feel that the more advanced topologies envisioned could be supported.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>