firmware: arm_scmi: Simplify with scoped for each OF child loop

Use scoped for_each_child_of_node_scoped() when iterating over device
nodes to make code a bit simpler.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Message-Id: <20240816151407.155034-1-krzysztof.kozlowski@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>