dt-bindings: soc: samsung: exynos-pmu: cleanup assigned clocks

"assigned-clocks" are not needed in the device schema as they come from
core schema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220706160257.27579-1-krzysztof.kozlowski@linaro.org