watchdog: (nv_tco) add support for MCP79

Tested on the Nvidia chipset with an SMBus controller PCI ID 0x0AA2
(as shown in the PCI listing during the boot sequence).

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>