dt-bindings: watchdog: renesas-wdt: Convert to json-schema

Convert the Renesas Watchdog Timer (WDT) Controller Device Tree binding
documentation to json-schema.

Add missing "renesas,r8a77980-wdt" compatible value.
Document missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Rob Herring <robh@kernel.org>