dt-bindings: power: supply: ab8500_fg: Add line impedance

To improve the inner resistance measurement of the battery we need
to account for the line impedance of the connector to the battery.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>