power: supply: max17042_battery: Add external_power_changed callback

If our supplier changes status, chances are we've changed status too,
let any listeners know about this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>