hw/arm/aspeed: Introduce aspeed_soc_cpu_type() helper

hw/arm/aspeed: Introduce aspeed_soc_cpu_type() helper

In order to alter AspeedSoCClass::cpu_type in the next
commit, introduce the aspeed_soc_cpu_type() helper to
retrieve the per-SoC CPU type from AspeedSoCClass.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>