hwmon: (pmbus/core) Add support for rated attributes

Adding implementation for new attributes (rated_min/rated_max) to
cover PMBus specification about rated values reporting:
MFR_VIN_MIN, MFR_VIN_MAX, MFR_IIN_MAX, MFR_PIN_MAX, MFR_VOUT_MIN,
MFR_VOUT_MAX, MFR_IOUT_MAX, MFR_POUT_MAX, MFR_MAX_TEMP_1/2/3.

Tested with OpenBMC stack. All rated attributes were available
and reported correct values.

Signed-off-by: Zbigniew Lukwinski <zbigniew.lukwinski@linux.intel.com>
Link: https://lore.kernel.org/r/1596224237-32280-4-git-send-email-zbigniew.lukwinski@linux.intel.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>