net: lan966x: Change the PTP pin used to read/write the PHC.

To read/write a value to a PHC, it is required to use a PTP pin.
Currently it is used pin 5, but change to pin 7 as is the last pin.
All the other pins will have different functions.

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