dmaengine: of: constify of_phandle_args in of_dma_find_controller()

of_dma_find_controller() does not modify passed of_phandle_args so the
argument can be made pointer to const for code safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240208202742.631307-2-krzysztof.kozlowski@linaro.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>