Documentation/bindings: net: ocelot: document the PTP ready IRQ

One additional interrupt needs to be described within the Ocelot device
tree node: the PTP ready one. This patch documents the binding needed to
do so.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mscc-ocelot.txt

@@ -17,9 +17,10 @@ Required properties:
   - "ana"
   - "portX" with X from 0 to the number of last port index available on that
     switch
-- interrupts: Should contain the switch interrupts for frame extraction and
-  frame injection
-- interrupt-names: should contain the interrupt names: "xtr", "inj"
+- interrupts: Should contain the switch interrupts for frame extraction,
+  frame injection and PTP ready.
+- interrupt-names: should contain the interrupt names: "xtr", "inj". Can contain
+  "ptp_rdy" which is optional due to backward compatibility.
 - ethernet-ports: A container for child nodes representing switch ports.
 
 The ethernet-ports container has the following properties
@@ -63,8 +64,8 @@ Example:
 			    "port2", "port3", "port4", "port5", "port6",
 			    "port7", "port8", "port9", "port10", "qsys",
 			    "ana";
-		interrupts = <21 22>;
-		interrupt-names = "xtr", "inj";
+		interrupts = <18 21 22>;
+		interrupt-names = "ptp_rdy", "xtr", "inj";
 
 		ethernet-ports {
 			#address-cells = <1>;