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

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

Musca boards use the embedded subsystems (SSE) tied to a specific
Cortex core. Our models only use the Cortex-M33.

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.

Remove the now unused MachineClass::default_cpu_type field.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240129151828.59544-7-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>