power: supply: ab8500: Standardize internal resistance

The nominal internal resistance isn't used by the AB8500
charging code, instead this resistance is measured continuously,
but we anyways migrate this to the standard property in
struct power_supply_battery_info.

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