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: I134027f28abd8e76ee8212685572272b1ddaa872
Signed-off-by: Andrew Dodd <atd7@cornell.edu>