net: lan966x: Add support for PTP_PF_EXTTS

Extend the PTP programmable pins to implement also PTP_PF_EXTTS
function. The PTP pin can be configured to capture only on the rising
edge of the PPS signal. And once an event is seen then an interrupt is
generated and the local time counter is saved.
The interrupt is shared between all the pins.

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