powerpc/85xx: Don't scan for TBI PHY addresses on MPC8569E-MDS boards

Sometimes (e.g. when there are no UEMs attached to a board)
fsl_pq_mdio_find_free() fails to find a spare address for a TBI PHY,
this is because get_phy_id() returns bogus 0x0000ffff values
(0xffffffff is expected), and therefore mdio bus probing fails with
the following message:

  fsl-pq_mdio: probe of e0082120.mdio failed with error -16

And obviously ethernet doesn't work after this.

This patch solves the problem by adding tbi-phy node into mdio node,
so that we won't scan for spare addresses, we'll just use a fixed one.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>