Rename A9SCUState::busdev field to parent_obj and turn realizefn into an
instance_init function to allow early MMIO mapping.

Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>