i8k: Support additional temperature sensors

The SMM API suggests that more than one temperature sensor is supported,
so add support for them. Currently only supported for hwmon interface.

Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>