coresight: do not probe if cpu is not online

If you use the parameter 'maxcpus=n', restrict boot time cpus to n.
To initialize the ETM on cpu, the cpus should be online. So if you restrict
boot time cpus, you can not enable the ETM on cpu which is rectriced(not
enabled). Skip the initialization of the ETM.

Change-Id: Idccf537ff4dce0ae5d1b60ecc1631180ad323fb8