This is question for the Turris team…
There is no collectd module for cpufreq in turris repository. Would you be so kind and add it? With recent change in kernel this is very tempting option 
Correct me if I am wrong but I thought I read somewhere that the package lists are going to be rebased in the upcoming 3.7 release. If that is the case and there is such a package in the OpenWRT/LEDE package lists it thus will be added.
Edit: found it.
Ah, that looks like a BIG task and BIG update 
In case someone needed, compiled collectd-mod-cpufreq_4.10.8-3_mvebu for Omnia
Luci changes: luci-app-statistics: add support for CPU frequency scaling stats · openwrt/luci@df3c872 · GitHub