perf, powerpc: Convert the FSL driver to use local64_t

For some reason the FSL driver got left out when we converted perf
to use local64_t instead of atomic64_t.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>