Documentation: dtb: lm87: Add hwmon binding documentation

This patch adds lm87 hwmon device tree node documentation.
Signed-off-by: Mahoda Ratnayaka <mahoda.ratnayaka@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/lm87.txt

+*LM87 hwmon sensor.
+
+Required properties:
+- compatible: Should be
+	"ti,lm87"
+
+- reg: I2C address
+
+optional properties:
+- has-temp3: This configures pins 18 and 19 to be used as a second
+             remote temperature sensing channel. By default the pins
+             are configured as voltage input pins in0 and in5.
+
+- has-in6: When set, pin 5 is configured to be used as voltage input
+           in6. Otherwise the pin is set as FAN1 input.
+
+- has-in7: When set, pin 6 is configured to be used as voltage input
+           in7. Otherwise the pin is set as FAN2 input.
+
+- vcc-supply: a Phandle for the regulator supplying power, can be
+              cofigured to measure 5.0V power supply. Default is 3.3V.
+
+Example:
+
+lm87@2e {
+	compatible = "ti,lm87";
+	reg = <0x2e>;
+	has-temp3;
+	vcc-supply = <&reg_5v0>;
+};