PM / devfreq: allow governors to use devfreq_get_freq_level

Level based governors may need to perform this lookup to
interpret the current frequency of the device.

Change-Id: Idf7246b05775a52f088c52b898d98fbab4fd942c
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>