PM: Fixed clockdomain state control for OMAP3

Hardware supervised control for clockdomain power state transitions now
enabled in omap3_pm_init(). Also fixed set_pwrdm_state() to allow state
changes between sleep states (i.e. RET<->OFF.)

Signed-off-by: Tero Kristo <tero.kristo@nokia.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>