MAINTAINERS: change tee mailing list

The old TEE mailing list tee-dev@lists.linaro.org is about to be
retired. From now on please use op-tee@lists.trustedfirmware.org
instead.

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