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 <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190503022958.1394-1-joel@jms.id.au
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>