dt-bindings: hwmon: Document ibm,cffps2 compatible string

Document the compatible string for version 2 of the IBM CFFPS PSU.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/1567192263-15065-2-git-send-email-eajames@linux.ibm.com


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>