power_supply: Add power_supply notifier

This patch adds a notifier chain to the power_supply, this helps drivers
in other subsystem to listen to changes in power supply subsystem.

This would help to take some actions in those drivers on changing the
power supply properties. One such scenario is to increase/decrease system
performance based on the battery capacity/voltage. Another scenario is to
adjust the h/w peak current detection voltage/current thresholds based on
battery voltage/capacity. The notifier helps drivers to listen to changes
in power_suppy susbystem without polling the power_supply properties

Signed-off-by: Jenny TC <jenny.tc@intel.com>
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Jenny TC <jenny.tc@intel.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>