PM/devfreq: Add bw_vbif governor

This governor should be used by KGSL driver to vote for msm bus
bandwidth using devbw devices.

KGSL driver needs to register a callback function that returns
required IB value, and call devfreq_vbif_update_bw() function
any time it want to vote for bus bandwidth.

Change-Id: I6c7c08b7c3255b3793f1868d9cde604175093145
Signed-off-by: Vladimir Razgulin <vrazguli@codeaurora.org>