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

Extend dt-bindings for lan966x with ptp external interrupt. This is
generated when an external 1pps signal is received on the ptp pin.

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