coresight: etm3x: Rearrange cp14 access detection

As we are about to refactor the platform specific handling,
move the DT property handling to generic helpers.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>