dt-bindings: mfd: gateworks-gsc: Add fan-tach mode

In 7497d4a6 ("hwmon: (gsc-hwmon) add fan sensor") a mode
was added to report RPM's from a fan tach input.

Add this mode to the dt-bindings for the Gateworks System Controller.
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/gateworks-gsc.yaml

@@ -83,8 +83,9 @@ properties:
                 2 - scaled voltage based on an optional resistor divider
                     and optional offset
                 3 - pre-scaled 16-bit voltage value
+                4 - fan tach input to report RPM's
             $ref: /schemas/types.yaml#/definitions/uint32
-            enum: [0, 1, 2, 3]
+            enum: [0, 1, 2, 3, 4]
 
           gw,voltage-divider-ohms:
             description: Values of resistors for divider on raw ADC input