coresight: Return error 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: I2d034ffc195c6f25adb1fb8b3413dbc323825bd6
Signed-off-by: Shaoqing Liu <shaoqingliu@codeaurora.org>