coresight: etm3x: adding cpu affinity to sysFS interface

Without access to the device tree, it is impossible to know
what CPU a tracer is affined to.  As such adding a new sysFS
interface to convey the information.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>