dt-bindings: watchdog: fsl-imx7ulp-wdt: Fix assigned-clock-parents

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

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220120172333.1628990-1-robh@kernel.org