hwmon: (pmbus) Add IBM Common Form Factor (CFF) power supply driver

Add the driver to monitor IBM CFF power supplies with hwmon over
pmbus.

Signed-off-by: Edward A. James <eajames@us.ibm.com>
[groeck: drop 'default n'; include bitops.h instead of jiffies.h]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>