MAINTAINERS: Add mailing list for the TEE subsystem

Add a mailing list for patch reviews and discussions related to TEE
subsystem.

Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>