watchdog: Add TCO support for nVidia chipsets

This driver adds support for /dev/watchdog for boards using either the MCP51 or
MCP55 chipsets.  These are also known as the nForce 430 and nForce 550.  This
driver is likely to work on other chipsets as well, though those are the only
two that have been tested.

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