net: lan966x: Add registers used to configure the PTP pin

Add registers that are used to configure the PTP pins. These registers
are used to enable the interrupts per PTP pin and to set the waveform
generated by the pin.

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