coresight: Make device to CPU mapping generic

The CoreSight components ETM and CPU-Debug are always associated
with CPUs. Replace the of_coresight_get_cpu() with a platform
agnostic helper, in preparation to add ACPI support.

Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>