powerpc: Fix regression introduced in r342771

In r342771, I introduced a regression in Power by abusing the platform
smp_topo() method as a shortcut for providing the MI information needed for
the stated sysctls. The smp_topo() method was already called later by
sched_ule (under the name cpu_topo()), and initializes a static array of
scheduler topology information. I had skimmed the smp_topo_foo() functions
and assumed they were idempotent; empirically, they are not (or at least,
detect re-initialization and panic).

Do the cleaner thing I should have done in the first place and add a
platform method specifically for core- and thread-count probing.

Reported by: luporl via jhibbits
Reviewed by: luporl
X-MFC-With: r342771
Differential Revision: https://reviews.freebsd.org/D18777