hw/arm/npcm7xx_boards: Simplify setting MachineClass::valid_cpu_types[]

hw/arm/npcm7xx_boards: Simplify setting MachineClass::valid_cpu_types[]

The npcm7xx Soc is created with a Cortex-A9 core, see in
hw/arm/npcm7xx.c:

  static void npcm7xx_init(Object *obj)
  {
      NPCM7xxState *s = NPCM7XX(obj);

      for (int i = 0; i < NPCM7XX_MAX_NUM_CPUS; i++) {
          object_initialize_child(obj, "cpu[*]", &s->cpu[i],
                                  ARM_CPU_TYPE_NAME("cortex-a9"));
      }

The MachineClass::default_cpu_type field is ignored: delete it.

Use the common code introduced in commit c9cf636d48 ("machine: Add
a valid_cpu_types property") to check for valid CPU type at the
board level.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240129151828.59544-8-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>