linux-phytec-fsl

PM / sleep: Update some system sleep documentation

Update some documentation related to system sleep to document new
features and remove outdated information from it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>