Documentation: trace: Add documentation for TRBE

Add documentation for the  TRBE under trace/coresight.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
[ Split from the TRBE driver patch ]
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20210405164307.1720226-20-suzuki.poulose@arm.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>