coresight: stm: Rearrange probing the stimulus area

As we are about to refactor the platform specific handling
re-arrange some of the DT specific property handling.

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>