3c59x: Add software timestamping

Added software timestamping ability. Tested with linuxptp and synchronized
clocks to an average of less than 200 microseconds on 10 megabit ethernet.

Tested on both Vortex and Boomerang models.

Signed-off-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>