watchdog: ixp4xx: Make sure restart always works

[ Upstream commit b4075ecf ]

The IXP4xx watchdog in early "A0" silicon is unreliable and
cannot be registered, however for some systems such as the
USRobotics USR8200 the watchdog is the only restart option,
so implement a "dummy" watchdog that can only support restart
in this case.

Fixes: 1aea5228

 ("watchdog: ixp4xx: Implement restart")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230926-ixp4xx-wdt-restart-v2-1-15cf4639b423@linaro.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>