coresight-etb: add CoreSight ETB driver

This driver manages CoreSight ETB (Embedded Trace Buffer) which
acts as a circular buffer sink collecting generated trace data.

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>