msm: msm_bus: Add support for setrate only clocks

Some clocks require that the driver only set a rate on them but not
actually enable the clocks. These are cases where the client wants to
directly control enabling/disabling the clock.

Change-Id: I98f4e1713daf6f72a17e09ccdc45a6367ed0f451
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>