Documentation: tee: Document TEE kernel interface

Update documentation with TEE bus infrastructure which provides an
interface for kernel client drivers to communicate with corresponding
Trusted Application.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Maxim Uvarov <maxim.uvarov@linaro.org>
Link: https://lore.kernel.org/r/1591253979-29067-1-git-send-email-sumit.garg@linaro.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>