PM: wakeup: Add routine to help fetch wakeup source object.

Some user might want to go through all registered wakeup sources
and doing things accordingly. For example, SoC PM driver might need to
do HW programming to prevent powering down specific IP which wakeup
source depending on. So add this API to help walk through all registered
wakeup source objects on that list and return them one by one.

Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Tested-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>