hw/char: avr: Add limited support for USART peripheral

hw/char: avr: Add limited support for USART peripheral

These were designed to facilitate testing but should provide enough
function to be useful in other contexts.  Only a subset of the functions
of each peripheral is implemented, mainly due to the lack of a standard
way to handle electrical connections (like GPIO pins).

[AM: Remove word 'Atmel' from filenames and all elements of code]
Suggested-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Michael Rolnik <mrolnik@gmail.com>
Signed-off-by: Sarah Harris <S.E.Harris@kent.ac.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[rth: Squash I/O size fix and file rename from f4bug]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Reviewed-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-20-huth@tuxfamily.org>