aspeed: add a per SoC mapping for the memory space

aspeed: add a per SoC mapping for the memory space

This will simplify the definition of new SoCs, like the AST2600 which
should use a slightly different address space and have a different set
of controllers.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20190618165311.27066-3-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>