power: supply: max17042_battery: Add support for the SCOPE property

Add support for the SCOPE property, always return SCOPE_SYSTEM,
as the max170xx is used for the main battery on all known systems
with a max170xx.

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>