driver-core: cpu: export total_cpus to fix CPU_FREQ_STAT=m build

Commit a2425ffd ("cpufreq: Persist cpufreq time in state data across
hotplug") from android-3.4 used total_cpus on the cpufreq stats module.
Unfortunately, this symbol is not exported, so build fails when
CPU_FREQ_STAT=m. This patch exports it for the time being to get the
tree building again.

Signed-off-by: Emilio López <turl@linux-sunxi.org>