davinci_emac: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the DaVinci EMAC driver. Tested together with the dp83640 PHY.

Cc: Anant Gole <anantgole@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>