[POWERPC] 4xx: Rename CPU nodes to avoid dtc incompatibility

Recent DTC versions disallow certain special characters in full paths without
being quoted with {}.  That however breaks compatibility with older DTC
versions.  Work around this by renaming the CPU nodes for the 4xx files to a
generic node name, and specify the processor type in the model property of the
CPU node.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>