dt-bindings: hwmon: (pwm-fan) Add tachometer interrupt

This adds the tachometer interrupt to the pwm-fan binding, which is
necessary for RPM support.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/pwm-fan.txt

@@ -8,6 +8,15 @@ Required properties:
 
 Optional properties:
 - fan-supply		: phandle to the regulator that provides power to the fan
+- interrupts		: This contains a single interrupt specifier which
+			  describes the tachometer output of the fan as an
+			  interrupt source. The output signal must generate a
+			  defined number of interrupts per fan revolution, which
+			  require that it must be self resetting edge interrupts.
+			  See interrupt-controller/interrupts.txt for the format.
+- pulses-per-revolution : define the tachometer pulses per fan revolution as
+			  an integer (default is 2 interrupts per revolution).
+			  The value must be greater than zero.
 
 Example:
 	fan0: pwm-fan {
@@ -38,3 +47,13 @@ Example:
 					};
 			     };
 		};
+
+Example 2:
+	fan0: pwm-fan {
+		compatible = "pwm-fan";
+		pwms = <&pwm 0 40000 0>;
+		fan-supply = <&reg_fan>;
+		interrupt-parent = <&gpio5>;
+		interrupts = <1 IRQ_TYPE_EDGE_FALLING>;
+		pulses-per-revolution = <2>;
+	};