coresight: etm4x: Add kernel configuration for CONTEXTID

Set the proper bit in the configuration register when contextID tracing
has been requested by user space.  That way PE_CONTEXT elements are
generated by the tracers when a process is installed on a CPU.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Robert Walker <robert.walker@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>