mfd: axp20x: Add supplied-from property to axp288_fuel_gauge cell

The power-supply framework has the notion of one power-supply device
being supplied by another. A typical example of this is a charger
charging a battery.

A tablet getting plugged in to charge (or plugged out) only results in
events seen by the axp288_charger device / MFD cell. Which means that
a change udev-event only gets send for the charger power-supply class
device, not for the battery (the axp288_fuel_gauge device).

The axp288_fuel_gauge does have an external_power_change'd callback
which will generate a change udev-event when called. But before this
commit this never got called because the power-supply core only calls
this when a power-supply class device's supplier changes and the
supplier link from axp288_charger to axp288_fuel_gauge was missing.

Add a "supplied-from" property to axp288_fuel_gauge cell, pointing
to the "axp288_charger" power-supply class device, so that the
axp288_fuel_gauge's external_power_change'd callback gets called on
axp288_charger state changes.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>