net: stmmac: Add Flexible PPS support

This adds support for Flexible PPS output (which is equivalent
to per_out output of PTP subsystem).

Tested using an oscilloscope and the following commands:

1) Start PTP4L:
	# ptp4l -A -4 -H -m -i eth0 &
2) Set Flexible PPS frequency:
	# echo <idx> <ts> <tns> <ps> <pns> > /sys/class/ptp/ptpX/period

Where, ts/tns is start time and ps/pns is period time, and ptpX is ptp
of eth0.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Vitor Soares <soares@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>