aspeed/soc: Add AST2600 support

Initial definitions for a simple machine using an AST2600 SoC (Cortex
CPU).

The Cortex CPU and its interrupt controller are too complex to handle
in the common Aspeed SoC framework. We introduce a new Aspeed SoC
class with instance_init and realize handlers to handle the differences
with the AST2400 and the AST2500 SoCs. This will add extra work to
keep in sync both models with future extensions but it makes the code
clearer.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20190925143248.10000-19-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>