aspeed: allocate RAM after the memory controller has checked the size

If the RAM size is invalid, the memory controller will use a default
value.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 1473438177-26079-15-git-send-email-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
