dt-bindings: power: supply: max17040: Add DT bindings for max17040 fuel gauge

Documentation of max17040 based fuel gauge characteristics.
For configure low level state of charge threshold alert signaled from
max17043/max17044 we add "maxim,alert-low-soc-level" property.
Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

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

+max17040_battery
+~~~~~~~~~~~~~~~~
+
+Required properties :
+ - compatible : "maxim,max17040" or "maxim,max77836-battery"
+ - reg: i2c slave address
+
+Optional properties :
+- maxim,alert-low-soc-level :	The alert threshold that sets the state of
+				charge level (%) where an interrupt is
+				generated. Can be configured from 1 up to 32
+				(%). If skipped the power up default value of
+				4 (%) will be used.
+- interrupts : 			Interrupt line see Documentation/devicetree/
+				bindings/interrupt-controller/interrupts.txt
+- wakeup-source :		This device has wakeup capabilities. Use this
+				property to use alert low SOC level interrupt
+				as wake up source.
+
+Optional properties support interrupt functionality for alert low state of
+charge level, present in some ICs in the same family, and should be used with
+compatible "maxim,max77836-battery".
+
+Example:
+
+	battery-fuel-gauge@36 {
+		compatible = "maxim,max77836-battery";
+		reg = <0x36>;
+		maxim,alert-low-soc-level = <10>;
+		interrupt-parent = <&gpio7>;
+		interrupts = <2 IRQ_TYPE_EDGE_FALLING>;
+		wakeup-source;
+	};