docs: staging/tee.rst: add a section on OP-TEE notifications

Adds a section on notifications used by OP-TEE, synchronous and
asynchronous.

Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>