ARM: dts: imx7ulp: Fix 'assigned-clocks-parents' typo

The correct property name is 'assigned-clock-parents', not
'assigned-clocks-parents'. Though if the platform works with the typo, one
has to wonder if the property is even needed.

Signed-off-by: Rob Herring <robh@kernel.org>
Fixes: 8b8c7d97 ("ARM: dts: imx7ulp: Add wdog1 node")
Signed-off-by: Shawn Guo <shawnguo@kernel.org>