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: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NOgnjen Galic <smclt30p@gmail.com>
Reviewed-by: NSebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>