[POWERPC] Cleanup SMT thread handling

This cleans up the SMT thread handling, removing some hard coded
assumptions and providing a set of helpers to convert between linux
cpu numbers, thread numbers and cores.

This implementation requires the number of threads per core to be a
power of 2 and identical on all cores in the system, but it's an
implementation detail, not an API requirement and so this limitation
can be lifted in the future if anybody ever needs it.

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