PM / devfreq: bw_hwmon: Add HW offload support to governor

Some HW monitors can do a better job of the sampling and the threshold
checking than the SW implementation in the governor.  Update the governor's
API to add support for them.

Change-Id: Id4b5593a5ed3290684ba43ebebe2466ba0b730b6
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: David Keitel <dkeitel@codeaurora.org>