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

The method getting TX timestamp by reading timestamp FIFO and
matching skbs list is common for DSA Felix driver too.
So move code out of ocelot_board.c, convert to use
ocelot_get_txtstamp() function and export it.

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>