Documentation: trace: Add documentation for Coresight Dummy Trace

Add documentation for Coresight Dummy Trace under trace/coresight.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Hao Zhang <quic_hazha@quicinc.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230602084149.40031-4-quic_hazha@quicinc.com