ppc4xx: Initialize magnetic couplers in PLU405

This patch fixes an ugly behavior of the IL712 magnetic couplers
as used on PLU405. These parts will remember their last state
over a power cycle which might cause unwanted behavior.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>