dt-binding: mediatek: watchdog: fix the description of compatible

The watchdog driver for MT2712 and MT8183 relies on DT data, so
the fallback compatible MT6589 won't work.

Signed-off-by: Crystal Guo <crystal.guo@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201014131936.20584-2-crystal.guo@mediatek.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>