watchdog: remove tango driver

The tango platform is getting removed, so the driver is no
longer needed.

Cc: Marc Gonzalez <marc.w.gonzalez@free.fr>
Cc: Mans Rullgard <mans@mansr.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Mans Rullgard <mans@mansr.com>
Link: https://lore.kernel.org/r/20210120162745.61268-5-arnd@kernel.org


[groeck: Removed devicetree bindings]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>