PM / devfreq: devbw: Add suspend/resume APIs

Absence of traffic is guaranteed when the device sitting behind a devbw
device is suspended. In such cases, it is a waste of power to make non-zero
bandwidth votes or to scale the devbw device. So, provide APIs to
suspend/resume the devbw device as needed.

Change-Id: Id58072aec7a9710eb917f248d9b9bd08d3a1ec6a
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>