net: lan966x: Add support for PTP_PF_PEROUT

Lan966x has 8 PTP programmable pins, where the last pins is hardcoded to
be used by PHC0, which does the frame timestamping. All the rest of the
PTP pins can be shared between the PHCs and can have different functions
like perout or extts. For now add support for PTP_FS_PEROUT.
The HW is not able to support absolute start time but can use the nsec
for phase adjustment when generating PPS.

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