coresight: document the bindings for the ATCLK

Put in a blurb in the device tree bindings indicating that
coresight blocks may have an optional ATCLK.

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