PM: runtime: add missed pm_request_autosuspend

The patch "PM / Runtime: Implement autosuspend support" introduces
"autosuspend" facility for runtime PM, but misses helper function
of pm_request_autosuspend, so add it.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>