PM / devfreq: Refactor CPUBW HWmon governor to be more generic

The refactored governor will be called bw_hwmon since it's no longer
limited to monitoring the CPU BW in hardware.

The refactor allows the governor to support multiple devfreq devices. This
is done by having different HW monitor instances register their capability
to monitor different devfreq devices and then picking the right HW monitor
based on which devfreq device is using this governor.

The refactor also fixes a very unlikely race condition that can happen when
starting/stopping the governor.

Change-Id: Ifeeeb256115f6ae3075f391d9b4b02666e06f737
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>