Documentation: coresight: Add PID tracing description

Documentation: coresight: Add PID tracing description

After support the PID tracing for the kernel in EL1 or EL2, the usage
gets more complicated.

This patch gives description for the PMU formats of contextID configs,
this can help users to understand how to control the knobs for PID
tracing when the kernel is in different ELs.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210206150833.42120-9-leo.yan@linaro.org
Link: https://lore.kernel.org/r/20210211172038.2483517-4-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>