[SPARC64]: Restore 2.4.x /proc/cpuinfo behavior for "ncpus probed" field.

Noticed by Tom 'spot' Callaway.

Even on uniprocessor we always reported the number of physical
cpus in the system via /proc/cpuinfo.  But when this got changed
to use num_possible_cpus() it always reads as "1" on uniprocessor.
This change was unintentional.

So scan the firmware device tree and count the number of cpu
nodes, and report that, as we always did.

Signed-off-by: David S. Miller <davem@davemloft.net>