coresight-etm4x: Read only access to the tracer's ID registers

ETM ID registers contain valuable information about the capabilities
of the implementation and are very useful when configuring the device for
various trace scenarios.

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