net: mscc: ocelot: convert to use ocelot_port_add_txtstamp_skb()

Convert to use ocelot_port_add_txtstamp_skb() for adding skbs which
require TX timestamp into list. Export it so that DSA Felix driver
could reuse it too.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>