net: lan966x: Add support for ptp clocks

The lan966x has 3 PHC. Enable each of them, for now all the
timestamping is happening on the first PHC.

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