hw/arm/cubieboard: restrict allowed RAM size to 512MiB and 1GiB

hw/arm/cubieboard: restrict allowed RAM size to 512MiB and 1GiB

The Cubieboard contains either 512MiB or 1GiB of onboard RAM [1].
Prevent changing RAM to a different size which could break user programs.

 [1] http://linux-sunxi.org/Cubieboard

Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200227220149.6845-4-nieklinnenbank@gmail.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>