target/avr: Add definitions of AVR core types

target/avr: Add definitions of AVR core types

AVR core types are:

  - avr5
  - avr51
  - avr6

Each core type covers multiple AVR MCUs, mentioned in the comments
before definition of particular AVR core type (part of this patch).

AVR core type defines shared features that are valid for all AVR
MCUs belonging in that type.

[AM: Split a larger AVR introduction patch into logical units]
Suggested-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Co-developed-by: Michael Rolnik <mrolnik@gmail.com>
Co-developed-by: Sarah Harris <S.E.Harris@kent.ac.uk>
Signed-off-by: Michael Rolnik <mrolnik@gmail.com>
Signed-off-by: Sarah Harris <S.E.Harris@kent.ac.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-9-huth@tuxfamily.org>
[PMD: Only include reviewed cores: avr5/avr51/avr6]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>