devfreq: Make cpubw_hwmon governor reusable and hardware agnostic

Since various hardware platforms might have different ways of monitoring
the CPU bandwidth, make the cpubw_hwmon governor hardware agnostic so that
it can be reused across these hardware platforms.

Change-Id: If7af01537dbf1df66551bea2bc73232d9e9264f7
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>