doc: Add coresight_cpu_debug.enable to kernel-parameters.txt

Add coresight_cpu_debug.enable to kernel-parameters.txt, this flag is
used to enable/disable the CPU sampling based debugging.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>