Add ATxmega common files and cpu definitions.
This works was originally developed by @Josar. The 2018 version
were port to 2021 mainline.
This version changes original port to have only the atxmega CPU
definition. With that, all family can be accomodated.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
1a88f0bad6