[CPUFREQ] acpi-cpufreq: add missing __percpu markup

acpi_perf_data is a percpu pointer but was missing __percpu markup.
Add it.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dave Jones <davej@redhat.com>