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>