USB: OTG: msm: Add debugfs entry to disable MSM BUS bandwidth voting

OTG driver requests for high bus bandwidth as long as USB cable is
connected to improve USB throughput. This vote causes scaling up of
fabric frequencies and increase in the over system power consumption.

Some users might not be interested in this feature or may want to
disable this at runtime for profiling the system performance with USB
cable attached.
Hence, add a debugfs entry to allow users to disable this voting.
By default this feature is enabled.

Command to disable BUS voting from USB:
echo "disable" > <debugfs>/msm_otg/bus_voting

Change-Id: Icdd9c56623a9c91ff3a130d80983c3ddeb0f91c4
CRs-fixed: 341452
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>