net: ethernet: ti: cpsw: Enable PHY timestamping

If the used PHYs also support hardware timestamping, all configuration requests
should be forwared to the PHYs instead of being processed by the MAC driver
itself.

This enables PHY timestamping in combination with the cpsw driver.

Tested with an am335x based board with two DP83640 PHYs connected to the cpsw
switch.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>