ast2400: add a memory controller device model

ast2400: add a memory controller device model

The uboot in the previous release of the SDK was using a hardcoded
value for memory size. This is not true anymore, the value is now
retrieved from the memory controller.

Below is a model for this device, only supporting unlock and
configuration. Without it, we endup running a guest with 64MB, which
is a bit low nowdays. It uses a 'silicon-rev' property and ram_size to
build a default value. Some bits should be linked to SCU strapping
registers but it seems a bit complex to add for the current need.

The model is ready for the AST2500 SOC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>