[POWERPC] 83xx: Make 83xx perfmon support selectable

Not all e300 cores support the performance monitors, and the ones
that don't will be confused by the mf/mtpmr instructions.  This
allows the support to be optional, so the 8349 can turn it off
while the 8379 can turn it on.  Sadly, those aren't config options,
so it will be left to the defconfigs and the users to make that
determination.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>