PM / devfreq: bw_hwmon: Add support for specifying count factor

Not all bandwidth monitors count in units of 1MB. Add a DT
property that indicates the number of bytes the monitor counts in
so we can properly scale the count we read from the hardware to
reflect the traffic. By default, we will assume 1MB units if the
property doesn't exist.

Change-Id: I1b581b2fffda04ba151df6e87221628368ff5b17
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>