power: supply: ab8500: Standardize voltages

The nominal voltage in this charge driver corresponds to
both the voltage_min_design_uv and voltage_max_design_uv
of struct power_supply_battery_info so assign both if this
is undefined.

The overcharge max voltage (when the charger should cut off)
is migrated at the same time so we move both voltages to
struct power_supply_battery_info.

Adjust the code to deal directly with the microvolt values
instead of converting them to millivolts.

Add *_uv suffixes for clarity and to make sure we have
changed all code sites using this member.

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