tg3: Enable GPHY APD on select devices

GPHY Autopowerdown (APD) is a way to save power when energy is not
detected on the wire.  At the moment, only the 5784 and 5761 are
capable of enabling this mode.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>