target/avr: Introduce basic CPU class object

target/avr: Introduce basic CPU class object

This patch introduces AVR CPU class object and its basic elements
and functions.

[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>
[thuth: Adjusted reset and parent_reset handling]
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-3-huth@tuxfamily.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>