hw/core/machine: Constify MachineClass::valid_cpu_types[]

hw/core/machine: Constify MachineClass::valid_cpu_types[]

Constify MachineClass::valid_cpu_types[i], as suggested by Richard
Henderson.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20231117071704.35040-2-philmd@linaro.org
[PMD: Constify HPPA machines,
      restrict valid_cpu_types to machine_class_init() handlers]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>