devfreq_devbw: Add support for voting for AB based on IB

Some generic devfreq governors might not provide AB values since that's a
devbw device specific attribute. In such cases, we might want to make an
average bandwidth (AB) vote that's a percentage of the IB vote to make
sure device BW requirement are not grossly misrepresented. This patch adds
support for that.

Change-Id: I76fbb8d688742058980f0d7568f2e7140023917e
Signed-off-by: Taniya Das <tdas@codeaurora.org>