coresight: refactor with function of_coresight_get_cpu

This is refactor to add function of_coresight_get_cpu(), so it's used to
retrieve CPU id for coresight component. Finally can use it as a common
function for multiple places.

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