fsl_pq_mdio: Revive Gianfar TBI PHY support

commit 1577ecef

 ("netdev: Merge UCC
and gianfar MDIO bus drivers") broke the TSEC TBI PHY support: the
driver now refuses to probe TBI MDIO buses as it doesn't know about
"fsl,gianfar-tbi" compatible entry, and thus _probe() fails with
-ENODEV status.

Fix this by adding "fsl,gianfar-tbi" to the list of known Gianfar
MDIO buses.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>