dt-binding: ptp_qoriq: document "fsl,extts-fifo" property

Documented "fsl,extts-fifo" property.
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/ptp/ptp-qoriq.txt

@@ -17,6 +17,8 @@ Clock Properties:
   - fsl,tmr-fiper1   Fixed interval period pulse generator.
   - fsl,tmr-fiper2   Fixed interval period pulse generator.
   - fsl,max-adj      Maximum frequency adjustment in parts per billion.
+  - fsl,extts-fifo   The presence of this property indicates hardware
+		     support for the external trigger stamp FIFO.
 
   These properties set the operational parameters for the PTP
   clock. You must choose these carefully for the clock to work right.