[POWERPC] Make default cputable entries reflect selected CPU family

Changes the cputable so that various CPU families that have an exclusive
CONFIG_ option have a more sensible default entry to use if the specific
processor hasn't been identified.

This makes the kernel more generally useful when booted on an unknown
PVR for things like new 4xx variants.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>