hw/arm/cubieboard: use ARM Cortex-A8 as the default CPU in machine definition

hw/arm/cubieboard: use ARM Cortex-A8 as the default CPU in machine definition

The Cubieboard is a singleboard computer with an Allwinner A10 System-on-Chip [1].
As documented in the Allwinner A10 User Manual V1.5 [2], the SoC has an ARM
Cortex-A8 processor. Currently the Cubieboard machine definition specifies the
ARM Cortex-A9 in its description and as the default CPU.

This patch corrects the Cubieboard machine definition to use the ARM Cortex-A8.

The only user-visible effect is that our textual description of the
machine was wrong, because hw/arm/allwinner-a10.c always creates a
Cortex-A8 CPU regardless of the default value in the MachineClass struct.

 [1] http://docs.cubieboard.org/products/start#cubieboard1
 [2] https://linux-sunxi.org/File:Allwinner_A10_User_manual_V1.5.pdf

Fixes: 8a863c8120994981a099
Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200227220149.6845-2-nieklinnenbank@gmail.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[note in commit message that the bug didn't have much visible effect]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>