hw/arm/aspeed: Check for CPU types in machine_run_board_init()

hw/arm/aspeed: Check for CPU types in machine_run_board_init()

Aspeed SoCs use a single CPU type (set as AspeedSoCClass::cpu_type).
Convert it to a NULL-terminated array (of a single non-NULL element).

Set MachineClass::valid_cpu_types[] to use the common machine code
to provide hints when the requested CPU is invalid (see commit
e702cbc19e ("machine: Improve is_cpu_type_supported()").

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: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>