dt-bindings: watchdog: migrate mt7621 text bindings to YAML

Soc Mt7621 Watchdog bindings used text format, so migrate them to YAML.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220926162549.805108-1-sergio.paracuellos@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>