hw/arm/fsl-imx: Add the cpu as child of the SoC object

Child properties form the composition tree. All objects need to be
a child of another object. Objects can only be a child of one object.

Respect this with the i.MX SoC, to get a cleaner composition tree.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190823143249.8096-5-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>