dt-bindings: mfd: max14577: Add reference to max14040_battery.txt descriptions

max77836 MFD has a fuel gauge that has a low SOC alert feature that is
described in:

  Documentation/devicetree/bindings/power/supply/max17040_battery.txt

Adding the reference to the documentation here.
Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
[Lee: Fix relative path]
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/max14577.txt

@@ -5,6 +5,8 @@ Battery Charger and SFOUT LDO output for powering USB devices. It is
 interfaced to host controller using I2C.
 
 MAX77836 additionally contains PMIC (with two LDO regulators) and Fuel Gauge.
+For the description of Fuel Gauge low SOC alert interrupt see:
+../power/supply/max17040_battery.txt
 
 
 Required properties: