net: dsa: microchip: ptp: add periodic output signal

LAN937x and KSZ PTP supported switches has Three Trigger output unit.
This TOU can used to generate the periodic signal for PTP. TOU has the
cycle width register of 32 bit in size and period width register of 24
bit, each value is of 8ns so the pulse width can be maximum 125ms.

Tested using ./testptp -d /dev/ptp0 -p 1000000000 -w 100000000 for
generating the 10ms pulse width

Signed-off-by: Christian Eggers <ceggers@arri.de>
Co-developed-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>