iop33x: update defconfig (default atu to on)

By default the iop3xx configurations are set to boot the platforms over
an nfs root configuration.  Since commit c34002c1 "iop: unconditionally
initialize the ATU on platforms known to be 'hosts'" this configuration
also requires iop3xx_init_atu=y to be specified on the kernel command
line.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>