sensors: Add enum value "CFM" to the Unit-property

The `DBus-sensors` package now implements the CFM sensor, and the
`xyz.openbmc_project.Sensor.Value` should provide the `CFM` at enum's of
"Unit"-property.

End-user-impact: The `xyz.openbmc_project.Sensor.Value.Unit` now
                 represents the 'CFM' type of CFM-sensors values

Signed-off-by: Igor Kononenko <i.kononenko@yadro.com>
Change-Id: I8b040ca4ca8589d2496677529c3020bf12cfe9bf
diff --git a/xyz/openbmc_project/Sensor/Value.interface.yaml b/xyz/openbmc_project/Sensor/Value.interface.yaml
index 91f356a..c9b4a55 100644
--- a/xyz/openbmc_project/Sensor/Value.interface.yaml
+++ b/xyz/openbmc_project/Sensor/Value.interface.yaml
@@ -42,6 +42,7 @@
           For objects in the power namespace, Unit must be "Watts".
           For objects in the energy namespace, Unit must be "Joules".
           For objects in the utilization namespace, Unit must be "Percent"
+          For objects in the airflow namespace, Unit must be "CFM"
 
 enumerations:
     - name: Unit
@@ -72,3 +73,6 @@
         - name: Percent
           description: >
             Resource utilization as percentage.
+        - name: CFM
+          description: >
+            To calculate Air Flow in Cubic Feet per Minute