hwmon: (nct6775) NCT6791D and NCT6792D have an additional temperature source

Both NCT6791D and NCT6792D permit selection of a 'virtual' temperature
register as temperature source. The virtual temperature registers are
registers 0xea to 0xef in bank 0 and can be written by software.

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