powerpc/8xxx: query feature reporting register for num cores on unknown cpus

doing so helps avant garde users, such as those using simulators that
allow users to configure the number of cores, so as to not have to
manually adjust u-boot sources.  h/w should also be reliably setting
FRR NCPU in the future.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>