net: dsa: mv88e6xxx: add rx/tx timestamping support

This patch implements RX/TX timestamping support.

The Marvell PTP hardware supports RX timestamping individual message
types, but for simplicity we only support the EVENT receive filter since
few if any clients bother with the more specific filter types.

checkpatch and reverse Christmas tree changes by Andrew Lunn.

Re-factor duplicated code paths and avoid IfOk anti-pattern, use the
common ptp worker thread from the class layer and time stamp UDP/IPv4
frames as well as Layer-2 frame by Richard Cochran.

Signed-off-by: Brandon Streiff <brandon.streiff@ni.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>