devfreq: add use_ab tunnable

In some usecases AB votes may not be needed but governor
cannot suppress the AB measured on HW.

Provide use_ab tunable for userspace to drop AB vote.

Change-Id: Ib7734e06fa00f86eb536b63526c16212012b614d
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>