dt-binding: ptp_qoriq: support fsl,tmr-fiper3 property

Add fsl,tmr-fiper3 property definition which is supported only
on DPAA2 and ENETC network controller hardware.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -18,6 +18,8 @@ Clock Properties:
   - fsl,tmr-add      Frequency compensation value.
   - fsl,tmr-fiper1   Fixed interval period pulse generator.
   - fsl,tmr-fiper2   Fixed interval period pulse generator.
+  - fsl,tmr-fiper3   Fixed interval period pulse generator.
+                     Supported only on DPAA2 and ENETC hardware.
   - 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.