ARM: OMAP3: clock: add API to enable/disable autoidle for a single clock

Some drivers require direct access to the autoidle functionality of the
interface clocks. Added clock APIs for these, so that the drivers do not
need to access CM registers directly.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>