PM debug: allow configurable wakeup from suspend on OMAP GPtimer

Using debugfs, export a configurable wakeup timer to be used to
wakeup system from suspend.

If a non-zero value is written to
/debug/pm_debug/wakeup_timer_seconds, A timer wakeup event will wake
the system and resume after the configured number of seconds.

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