arm: aspeed: Set SDRAM size

arm: aspeed: Set SDRAM size

We currently use Qemu's default of 128MB. As we know how much ram each
machine ships with, make it easier on users by setting a default.

It can still be overridden with -m on the command line.

Signed-off-by: Joel Stanley <[email protected]>
Reviewed-by: Andrew Jeffery <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20190503022958[email protected]
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>