net: dsa: microchip: ptp: add support for perout programmable pins

There are two programmable pins available for Trigger output unit to
generate periodic pulses. This patch add verify_pin for the available 2
pins and configure it with respect to GPIO index for the TOU unit.

Tested using testptp
./testptp -i 0 -L 0,2
./testptp -i 0 -d /dev/ptp0 -p 1000000000
./testptp -i 1 -L 1,2
./testptp -i 1 -d /dev/ptp0 -p 100000000

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>