TEE driver code and documentation

Replace the current DebugFS and custom ioctl based FF-A user-space
interface with an implementation which uses the TEE subsystem.
Currently the new TEE driver supports Trusted Services specific
protocol and enables communication with PSA Secure Partitions. The
design allows extending the TEE driver with other protocols in the
future when demand arises.

The main intention of the change is to break away from the
proof-of-concept nature of the previous implementation, coming from
the debug nature of DebugFS (it is disabled in release builds), and
the foreseeable challenges of up-streaming a new user-space API.

Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Change-Id: If2a59d5ca15eebeb30dad229fb99207aadc13d93