power: supply: max17042_battery: Add support for the STATUS property

Userspace prefers the driver having a status property over having to guess
itself. Specifically this will properly make the GNOME3 UI (and likely
others) properly show discharging / charging / full status, instead
of always showing discharging as status.

Note that in the case there is no charger driver supplying the max17042,
then a status of unknown will get returned. At least upower treats
this the same as not having a status attribute, so in this case nothing
changes from a userspace pov.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>