net: ethernet: ti: cpsw: allow PTP 224.0.0.107 to be timestamped

Tested on AM572x with cpsw v1.15 for PTP sync and delay_req messages.
It doesn't work on cpsw v1.12, so added only for cpsw v > 1.15.

Command for testing:
ptp4l -P -4 -H -i eth0 -l 6 -m -q -p /dev/ptp0 -f ptp.cfg
where ptp.cfg:

[global]
tx_timestamp_timeout     20

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>