powerpc/83xx: Move gianfar mdio nodes under the ethernet nodes

Currently it doesn't matter where the mdio nodes are placed, but with
power management support (i.e. when sleep = <> properties will take
effect), mdio nodes placement will become important: mdio controller
is a part of the ethernet block, so the mdio nodes should be placed
correctly. Otherwise we may wrongly assume that MDIO controllers are
available during sleep.

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