coresight-etm4x: Read only access to the main management registers

Having access to the ETMv4 management registers is very useful as they
give meaningful information on how the IP block has been configured at
synthesis time.

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