Add ENET/Gbps Ethernet support to FEC device

The ENET device (present in i.MX6) is "derived" from FEC and backward
compatible with it.

This patch adds the necessary support of the added feature in the ENET
device to allow Linux to use it (on supported processors).

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>