dt-bindings: power: Introduce one property to present the battery internal resistance

The internal resistance of a battery is not a constant in its life cycle,
this varies over the age of the battery or temperature and so on. But we
just want use one constant battery internal resistance to estimate the
battery capacity. Thus this patch introduces one property to present
the battery factory internal resistance for battery information.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>