net: lan966x: Add support for ptp interrupts

When doing 2-step timestamping the HW will generate an interrupt when it
managed to timestamp a frame. It is the SW responsibility to read it
from the FIFO.

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