coresight-etm4x: Controls pertaining to the counter functions

Adding sysfs entries related to the counter functionality, more
specifically to set, control and reload the counters.

Signed-off-by: Pratik Patel <pratikp@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>