hw/avr: Add some ATmega microcontrollers

hw/avr: Add some ATmega microcontrollers

Add some AVR microcontrollers from the ATmega family:

  - middle range: ATmega168 and ATmega328
  - high range: ATmega1280 and ATmega2560

For product comparison:
  https://www.microchip.com/wwwproducts/ProductCompare/ATmega168P/ATmega328P
  https://www.microchip.com/wwwproducts/ProductCompare/ATmega1280/ATmega2560

Datasheets:
  http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf
  http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf

[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>
[thuth: Rebased to master, fixed object_initialize_child() calls etc.]
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-25-huth@tuxfamily.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>