power: supply: ab8500: Respect charge_restart_voltage_uv

The battery info contains a voltage indicating when the voltage
is so low that it is time to restart the CC/CV charging.
Make the AB8500 respect and prioritize this setting over the
hardcoded 95% threshold.

Break out the check into its own function and add some safeguards
so we do not run into unpredictable side effects.

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