powerpc/pseries: Pass more accurate number of supported cores to firmware

Updated variant of a patch by Joel Schopp.

The field containing the number of supported cores which we pass to
firmware via the ibm,client-architecture call was set by a previous
patch statically as high as is possible (NR_CPUS).

However, that value isn't quite right for a system that supports
multiple threads per core, thus permitting the firmware to assign
more cores to a Linux partition than it can really cope with.

This patch improves it by using the device-tree to determine the
number of threads supported by the processors in order to adjust
the value passed to firmware.

Signed-off-by: Joel Schopp <jschopp@austin.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>