OMAP: PM: Add new sysfs option for disabling clocks when entering idle

There are drivers that are not disabling their clocks (gpio &
uart). These clocks need to be disabled if retention/off state is
wanted when idling. Before disabling them in idle loop this option
needs to be checked.

Signed-off-by: Jouni Hogander <jouni.hogander@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>