target/avr: Convert to 3-phase reset

target/avr: Convert to 3-phase reset

Convert the avr CPU class to use 3-phase reset, so it doesn't
need to use device_class_set_parent_reset() any more.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20221124115023.2437291-4-peter.maydell@linaro.org