hwmon: (pmbus) Auto-detect temp2 and temp3 registers/attributes

Additional temperature attribute support is easy to detect, so do it.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
Cc: stable.kernel.org # 2.6.39