powerpc/83xx: Enable FIXED_PHY in mpc834x_itx and mpc83xx defconfigs

This is needed so that Vitesse 7385 5-port switch could work on
MPC8349E-mITX boards.

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