hw/arm/stm32f205: Report error when incorrect CPU is used

hw/arm/stm32f205: Report error when incorrect CPU is used

The 'netduino2' machine ignores the CPU type requested by the
command line. This might confuse users, since the following will
create a machine with a Cortex-M3 CPU:

  $ qemu-system-arm -M netduino2 -cpu cortex-a9

Set the MachineClass::valid_cpu_types field (introduced in commit
c9cf636d48 "machine: Add a valid_cpu_types property").
Remove the now unused MachineClass::default_cpu_type field.

We now get:

  $ qemu-system-arm -M netduino2 -cpu cortex-a9
  qemu-system-arm: Invalid CPU type: cortex-a9-arm-cpu
  The valid types are: cortex-m3-arm-cpu

Since the SoC family can only use Cortex-M3 CPUs, hard-code the
CPU type name at the SoC level, removing the QOM property
entirely.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-id: 20231117071704.35040-4-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>