hw/avr: Introduce few Arduino boards

Hi,

This series add the arduino boards, aiming at removing the
'sample' board that doesn't follow any specification.

Since v3:
- Rebased on Michael's v41
- Drop 'extram' unused field (Igor)
- Renamed devices AVR -> Atmel (Aleksandar)
  (I haven't renamed structure names to ease review)

Since v2:
- rebased on Michael's v40

Since v1:
- Addressed Igor comments
- Addressed Aleksandar comments
- Fixed UART issue (was due to IRQ shifted by 2 in CPU)

Since Michael's work is not yet merged, Various of my patches
- which are trivials or simple renames - could be squashed
directly on his patches, if we ever care.
[I believe sending this patches is easier/quicker than keeping
asking Michael to respin his series infinitely].

Michael, do you mind testing it? The full series is available
here: https://gitlab.com/philmd/qemu/commits/arduino-v4

Regards,

Phil.

Obsoletes: <20191229224505.24466-1-f4bug@amsat.org>
Based-on: <20200118191416.19934-1-mrolnik@gmail.com>
https://www.mail-archive.com/qemu-devel@nongnu.org/msg671707.html