power: add to_power_supply macro to the API

This patch adds the to_power_supply macro to upcast
a device to a power_supply struct.

This is needed because the same piece of code using
container_of is used in various other places, so we
abstract away such low-level operations via a macro.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ognjen Galic <smclt30p@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>