power: supply: ab8500: Standardize design capacity

Now that we know that we have only one battery type to
deal with we can proceed to transfer properties to
struct power_supply_battery_info.

The designed capacity for the battery was in a custom field
of the custom battery type in mAh, transfer this to the
standard charge_full_design_uah property in
struct power_supply_battery_info and augment the code
accordingly.

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