MAINTAINERS: Mark the OP-TEE mailing list moderated

MAINTAINERS: Mark the OP-TEE mailing list moderated

After sending a patch to op-tee@lists.trustedfirmware.org, I got the
typical response for a moderated list:

    Your mail to 'op-tee@lists.trustedfirmware.org' with the subject

	[...]

    Is being held until the list moderator can review it for approval.

    The message is being held because:

	The message is not from a list member

    Either the message will get posted to the list, or you will receive
    notification of the moderator's decision.

Mark this mailing list moderated in MAINTAINERS.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>