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

At least upower prefers the more precise charge_now sysfs value over
capacity and the max17042 has the info, so lets export it.

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>