powerpc/4xx: Disable PCI_LEGACY

The single board defconfig files were missed during the cleanup
of CONFIG_PCI_LEGACY in the multi-board config files.  This
disables the option for the single board configs, as it isn't
used by anything for these boards.

Reported-by: Cheng Renquan <crquan@gmail.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>