coresight: documentation for coresight framework and drivers

Documentation containing an explanation on what the framework
provides and the drivers working with it.  A minimal example
on how to use the functionality is also provided.
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>