[POWERPC] Initialize all mv64x60 devices even if one fails

If one of the devices of the mv64x60 init fails, the remaining
devices are not initialized.

This changes the code to display an error and continue the
initialization.

Signed-off-by: Remi Machet <(rmachet@slac.stanford.edu)>
Signed-off-by: Paul Mackerras <paulus@samba.org>