hwmon: (nct6775) Add support for multiple virtual temperature sources

For virtual temperatures, the actual temperature values are written
by software, presumably by the BIOS. This functionality is (as of
right now) supported on NCT6791D, NCT6792D, and NCT6793D. On those chips,
the temperatures are written into registers 0xea..0xef on page 0.
This is known to be used on some Asus motherboards, where the actual
temperature source can be configured in the BIOS.

Report the 'virtual' temperatures for all monotoring sources to address
this situation.

Example for the resulting output (as seen with the 'sensors' command):

nct6791-isa-0290
Adapter: ISA adapter
...
Virtual_TEMP:           +31.0°C
PECI Agent 0:           +38.5°C
Virtual_TEMP:           +32.0°C
...

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