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 <[email protected]>
Reported-by: Andrew Jones <[email protected]>
Tested-by: Niek Linnenbank <[email protected]>
Message-id: 20200303091254[email protected]
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>