hw/avr: Add limited support for some Arduino boards

hw/avr: Add limited support for some Arduino boards

Arduino boards are build with AVR chipsets. Add some of these
boards:

  - Arduino Duemilanove
  - Arduino Uno
  - Arduino Mega

For more information:
  https://www.arduino.cc/en/Main/Products
  https://store.arduino.cc/arduino-genuino/most-popular

[AM: Remove word 'Atmel' from filenames and all elements of code]
Suggested-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Joaquin de Andres <me@xcancerberox.com.ar>
[thuth: sysbus_init_child_obj() ==> object_initialize_child()]
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-26-huth@tuxfamily.org>