coresight: Affinity to invalid if no cpu nodes are found

By default if no node found the function of_coresight_get_cpu return CPU0
which will cause problem when CPU0 etm probe.So should return invalid value
in case of no nodes found.

Change-Id: I9f71a0e54d9980049a8ee606ca8b2c51e1e66740
Signed-off-by: Mulu He <muluhe@codeaurora.org>