net: dsa: mv88e6xxx: Enable PTP receive for mv88e6390

The switch receives management traffic such as STP and LLDP. However, PTP
messages are not received, only transmitted.

Ideally, the switch would trap all PTP messages to the management CPU. This
particular switch has a PTP block which identifies PTP messages and traps them
to a dedicated port. There is a register to program this destination. This is
not used at the moment.

Therefore, program it to the same port as the MGMT traffic is trapped to. This
allows to receive PTP messages as soon as timestamping is enabled.

In addition, the datasheet mentions that this register is not valid e.g., for
6190 variants. So, add a new PTP operation which is added for the 6390 and 6290
devices.

Tested simply like this on Marvell 88E6390, revision 1:

|/ # ptp4l -2 -i lan4 --tx_timestamp_timeout=40 -m
|[...]
|ptp4l[147.450]: master offset         56 s2 freq   +1262 path delay       413
|ptp4l[148.450]: master offset         22 s2 freq   +1244 path delay       434
|ptp4l[149.450]: master offset          5 s2 freq   +1234 path delay       446
|ptp4l[150.451]: master offset          3 s2 freq   +1233 path delay       451
|ptp4l[151.451]: master offset          1 s2 freq   +1232 path delay       451
|ptp4l[152.451]: master offset         -3 s2 freq   +1229 path delay       451
|ptp4l[153.451]: master offset          9 s2 freq   +1240 path delay       451

Link: https://lore.kernel.org/r/CAFSKS=PJBpvtRJxrR4sG1hyxpnUnQpiHg4SrUNzAhkWnyt9ivg@mail.gmail.com


Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>