include: trace: Add SCMI header with trace events

Adding trace events would help to measure the speed of the communication
channel. It can be also potentially used helpful during investigation
of some issues platforms which use different transport layer.

Update also MAINTAINERS file with information that the new trace events
are maintained.

Suggested-by: Jim Quinlan <james.quinlan@broadcom.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>