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 <[email protected]>
Co-developed-by: Michael Rolnik <[email protected]>
Co-developed-by: Sarah Harris <[email protected]>
Signed-off-by: Michael Rolnik <[email protected]>
Signed-off-by: Sarah Harris <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Acked-by: Igor Mammedov <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <20200705140315[email protected]>
[PMD: Only include reviewed cores: avr5/avr51/avr6]
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>