dt-bindings: hwmon: Document ibm,cffps compatible string

Document this string that indicates that any version of the power supply
may be connected. In this case, the driver must detect the version
automatically.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1570648262-25383-2-git-send-email-eajames@linux.ibm.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ibm,cffps1.txt

@@ -5,6 +5,9 @@ Required properties:
  - compatible				: Must be one of the following:
 						"ibm,cffps1"
 						"ibm,cffps2"
+						or "ibm,cffps" if the system
+						must support any version of the
+						power supply
  - reg = < I2C bus address >;		: Address of the power supply on the
 					  I2C bus.