hw/arm: Use sysbus_init_child_obj for correct reference counting

Both object_initialize() and qdev_set_parent_bus() increase the
reference counter of the new object, so one of the references has
to be dropped afterwards to get the reference counting right.
In machine model code this refcount leak is not particularly
problematic because (unlike devices) machines will never be
created on demand via QMP, and they are never destroyed.
But in any case let's use the new sysbus_init_child_obj() instead
to get the reference counting here right.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190823143249.8096-4-philmd@redhat.com
[PMM: rewrote commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>