[POWERPC] Add PMC type to cputable

Add cputable entries for which type of PMC implementation the processor
has.

I've only filled in the current 64-bit processors, the unfilled default
value will have same behaviour as before so it can be done over time
as needed.

Also tidy up the dummy_perf implementation a bit, aggregating it into
one function with ifdefs instead of several.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>