net: dsa: mv88e6xxx: add workaround for 6341 timestamping

88E6341 devices default to timestamping at the PHY, but due to a
hardware issue, timestamps via this component are unreliable. For
this family, configure the PTP hardware to force the timestamping
to occur at the MAC.

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