documentation: watchdog: remove documentation for ixp2000

The ixp2000 watchdog driver was removed in commit 065e8238 ("watchdog: remove ixp2000 driver")

No need to keep its documentation, so remove it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>