PM / devfreq: Add ARM PMU support for bw_hwmon governor

The ARM PMU supports monitoring bus access from each CPU. It also has the
ability to raise an IRQ when the counters overflow. This allows for it to
be used with the bw_hwmon governor to scale the CPU BW requests by
monitoring on the actual bus access traffic.

Change-Id: I0594a6acb846acdc11a18744033636951f22e387
Signed-off-by: Jacob Stevens <jstevens@codeaurora.org>