net: sparx5: Add support for ptp clocks

The sparx5 has 3 PHC. Enable each of them, for now all the
timestamping is happening on the first PHC.

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