PM / devfreq: bw_hwmon: Split out sw and hw paths

Let's split out the sw and hw counter configuration code paths
indicated by has_hw_sampling() into inline functions for the two
different types of monitors. This allows us to add different
types of monitors in the future with minimal changes.

Change-Id: I5cf6a1fe4d84ee0958fe68601cb1e76836d10256
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>