watchdog: Add support for sp5100 chipset TCO

This driver adds /dev/watchdog support for the AMD sp5100 aka SB7x0 chipsets.

It follows the same conventions found in other /dev/watchdog drivers.

Signed-off-by: Priyanka Gupta <priyankag@google.com>
Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>