dt-bindings: watchdog: Add watchdog yaml file for Sunplus SP7021

This adds the documentation for the devicetree bindings of the Sunplus
SP7021 watchdog driver, found from SP7021 SoCs and newer.

Signed-off-by: Xiantao Hu <xt.hu@cqplus1.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220324031805.61316-2-xt.hu@cqplus1.com


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