hwmon: (lm90) Add support for GMT G781

GMT G781 is a ADM1032-compatible temperature sensor chip.
Add support to the LM90 driver.

Cc: Mike Gorchak <lestat@i.com.ua>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>