power_supply: Add API for safe access of power supply function attrs

Add simple wrappers for accessing power supply's function attributes:
 - get_property -> power_supply_get_property
 - set_property -> power_supply_set_property
 - property_is_writeable -> power_supply_property_is_writeable
 - external_power_changed -> power_supply_external_power_changed

This API along with atomic usage counter adds a safe way of accessing a
power supply from another driver. If power supply is unregistered after
obtaining reference to it by some driver, then the API wrappers won't be
executed in invalid (freed) context.

Next patch changing the ownership of power supply class is still needed
to fully fix race conditions in accessing freed power supply.

CRs-Fixed: 2076000
Change-Id: Ic5f18ecd94dc6ec6aafc584b303c514609a9ada0
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>
Git-commit: bc1540561c9ede1efb6d7bf44804676d3d02a3cc
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git


Signed-off-by: Ankit Sharma <ansharma@codeaurora.org>