[POWERPC] 83xx: MPC837xRDB's VSC7385 ethernet switch isn't on the MDIO bus

MDIO-less PHYs should use CONFIG_FIXED_PHY driver and appropriate
fixed-link property in the device tree.

If not, ethernet will not work:
  e0024520:03 not found
  eth1: Could not attach to PHY
  IP-Config: Failed to open eth1

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