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>

Documentation/devicetree/bindings/arm/coresight.txt

@@ -21,11 +21,14 @@ its hardware characteristcs.
 	* reg: physical base address and length of the register
 	  set(s) of the component.
 
-	* clocks: the clock associated to this component.
-
-	* clock-names: the name of the clock as referenced by the code.
-	  Since we are using the AMBA framework, the name should be
-	  "apb_pclk".
+	* clocks: the clocks associated to this component.
+
+	* clock-names: the name of the clocks referenced by the code.
+	  Since we are using the AMBA framework, the name of the clock
+	  providing the interconnect should be "apb_pclk", and some
+	  coresight blocks also have an additional clock "atclk", which
+	  clocks the core of that coresight component. The latter clock
+	  is optional.
 
 	* port or ports: The representation of the component's port
 	  layout using the generic DT graph presentation found in