hwmon: (nct6775) Monitor additional temperature registers

The number of SMIOVT registers on NCT6779 and NCT6791 is limited to 2.
As result, the driver may not report some of the temperatures used
for fan control. This can result in some of the pwmX_temp_sel or
pwm2_weight_temp_sel attributes to wrongly return 0.
Fortunately, the chip has registers to monitor those temperatures.
Add them to the list of temperatures to report.

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