luci-app-statistics: add support for CPU frequency scaling stats

Add support for 'cpufreq' plugin in collectd, which monitors
the CPU cores' frequencies. Some platforms enable dynamic
frequency scaling according to CPU load.

'cpufreq' plugin can currently be built for x86 and mvebu.
(and it should likely be enabled also for ipq806x)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>