power: supply: axp288_fuelguage: Do not bind when the fg function is not used

Some devices with an AXP288 PMIC do not have a battery at all, or use
external charger and fuelgauge ICs instead of the AXP288 builtin
functionality.

On such devices we should not bind to the fuelgauge function to avoid
exporting a non working power_supply class device.

This also avoids the following errors repeating over and over again in
dmesg:

axp288_fuel_gauge axp288_fuel_gauge: capacity measurement not valid
axp288_fuel_gauge axp288_fuel_gauge: Error 0xe2 contents not valid
power_supply axp288_fuel_gauge: driver failed to report 'charge_now'
property: -6

Cc: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>