dt-bindings: watchdog: Add compatible for Mediatek MT7986

This commit adds dt-binding documentation of watchdog for Mediatek MT7986
SoC Platform.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210726071439.14248-11-sam.shih@mediatek.com


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