cpufreq: conservative: add support for micro idle accounting

Originally, mininum sample rate calculations assume that micro idle accounting is non-existent. This is incorrect. Thus, we can afford to set the minimum sample rate to lower levels. This should improve performance on conservative governor by a respectable amount.

With a more accurate and sensitive sample rates, we can afford to loosen up on frequency thresholds.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>