coresight: etm: Fail probe for unsupported CPUs

Fail probe for CPUs which are not supported by the kernel
else we end up trying to configure ETM on different CPU
while executing on CPU0. This results in a NOC error.

Change-Id: I0f7917cfe87dbe4f6a2ab8ba6166d5b052e3c4fa
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>