dt-bindings: net: lan966x: Extend with the ptp interrupt

Extend dt-bindings for lan966x with ptp interrupt. This is generated
when doing 2-step timestamping and the timestamp can be read from the
FIFO.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>