power_supply: Add driver private data

Allow drivers to store private data inside power_supply structure for
later usage in power supply operations.

Usage of driver private data is necessary to access driver's state
container object from power supply calls (like get_property()) if struct
'power_supply' is a stored there as a pointer, for example:

struct some_driver_info {
	struct i2c_client       *client;
	struct power_supply     *power_supply;
	...
}

In such case one cannot use container_of() and must store pointer to
state container as private data.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sebastian Reichel <sre@kernel.org>