powerpc/dts/fsl: t2080rdb: use the Cortina PHY driver compatible

The Cortina PHY is not compatible with IEEE 802.3 clause 45.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
[scottwood: made commit message about compatibility, not driver choice]
Signed-off-by: Scott Wood <oss@buserror.net>