net: phy: Add support for 1PPS out and external timestamps

The perout function is used to generate a 1PPS signal, synchronized
to the PHC.  This is accomplished by a using the hardware oneshot
functionality, which is reset by a timer.

The external timestamp function is set up for a 1PPS input pulse,
and uses a timer to poll for temestamps.

Both functions use the SYNC_OUT/SYNC_IN1 pin, so cannot run
simultaneously.

Co-developed-by: Lasse Johnsen <l@ssejohnsen.me>
Signed-off-by: Lasse Johnsen <l@ssejohnsen.me>
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>