dt-bindings: watchdog: Add YAML schemas for the generic watchdog bindings

The watchdogs have a bunch of generic properties that are needed in a
device tree. Add a YAML schemas for those.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190821143835.7294-1-mripard@kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>