hw/arm/cubieboard: make sure SOC object isn't leaked

hw/arm/cubieboard: make sure SOC object isn't leaked

SOC object returned by object_new() is leaked in current code.
Set SOC parent explicitly to board and then unref to SOC object
to make sure that refererence returned by object_new() is taken
care of.

The SOC object will be kept alive by its parent (machine) and
will be automatically freed when MachineState is destroyed.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reported-by: Andrew Jones <drjones@redhat.com>
Tested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200303091254.22373-1-imammedo@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>