ucc_geth: fix non-functional fixed phy support

The new Fixed PHY method, fixed-link property, isn't
impl. for ucc_geth which makes fixed PHYs non functional.
Add support for the new method to restore the Fixed PHY
functionality.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>