powerpc/mpc8313erdb: fix kernel panic because mdio device is not probed

Probe the new mdio node added by b31a1d8b.  Fix kernel panic problem when
gianfar driver wants to get the of_platform_device of that mdio.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>