OMAP3: PM debug: allow runtime toggle of PM features

Allow enable/disable of low-power states during idle.  To
enable low-power idle:

   echo 1 > /debug/pm_debug/sleep_while_idle

 to disable:

   echo 0 > /debug/pm_debug/sleep_while_idle

Also allow enable/disable of OFF-mode.  To enable:

   echo 1 > /debug/pm_debug/enable_off_mode

 to disable:

   echo 0 > /debug/pm_debug/enable_off_mode

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>