dt-bindings: power: supply: max17042: describe interrupt

The Maxim 17042-family of fuel gauges are often embedded in other Maxim
chips, e.g. in Maxim 77693 which is a companion power management IC.
In such designs there might be actually two interrupts:
 - INTB signaling change from charger, flash or MUIC,
 - ALERT signaling change from fuel gauge.

Describe the interrupt in bindings to make it clear it is about the fuel
gauge ALERT interrupt, not the INT.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>