powerpc/85xx: Fix ethernet link detection on MPC8569E-MDS boards

Linux isn't able to detect link changes on ethernet ports that were
used by U-Boot. This is because U-Boot wrongly clears interrupt
polarity bit (INTPOL, 0x400) in the extended status register (EXT_SR,
0x1b) of Marvell PHYs.

There is no easy way for PHY drivers to know IRQ line polarity (we
could extract it from the device tree and pass it to phydevs, but
that'll be quite a lot of work), so for now just reset the PHYs to
their default states.

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