Support LAN743x PTP periodic output on any GPIO

The LAN743x Ethernet controller provides two independent PTP event
channels. Each one can be used to generate a periodic output from
the PTP clock. The output can be routed to any one of the available
GPIO pins on the device.

The PTP clock API can now be used to:
- select any LAN743x GPIO pin to function as a periodic output
- select either LAN743x PTP event channel to generate the output

The LAN7430 has 4 GPIO pins that are multiplexed with its internal
PHY LED control signals. A pin assigned to the LED control function
will be assigned to the GPIO function if selected for PTP periodic
output.

Signed-off-by: John Efstathiades <john.efstathiades@pebblebay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>