target/avr: Use generic cpu_list()

target/avr: Use generic cpu_list()

Before it's applied:

[gshan@gshan q]$ ./build/qemu-system-avr -cpu ?
avr5-avr-cpu
avr51-avr-cpu
avr6-avr-cpu

After it's applied:

[gshan@gshan q]$ ./build/qemu-system-avr -cpu ?
Available CPUs:
  avr5
  avr51
  avr6

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20231114235628.534334-10-gshan@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>