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

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>