cpufreq_conservative: Improve support for micro idle accounting

The minimum samplerate calculations of the current conservative
implementation assume that there is not micro idle accounting present,
however, we do have that capability in our idle estimates.  As a result,
we can support lower polling intervals just like ondemand does.

Change-Id: I183d494a74a99d4c662554895f9eb6d9f4092162
Signed-off-by: Andrew Dodd <atd7@cornell.edu>