dt-bindings: watchdog: ti,rti-wdt: Fix assigned-clock-parents

With 'unevaluatedProperties' support implemented, the ti,rti-wdt example
has the following warning:

/home/rob/proj/git/linux-dt/.build-arm64/Documentation/devicetree/bindings/watchdog/ti,rti-wdt.example.dt.yaml: watchdog@2200000: Unevaluated properties are not allowed ('assigned-clock-parents' was unexpected)

The problem is the schema has a typo in 'assigned-clocks-parents'. As
it is not required to list assigned clocks in bindings, just drop the
property definitions to fix this.

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: linux-watchdog@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211206174028.2294330-1-robh@kernel.org