The Pi 3A+ is a stripped down version of the 3B:
- 512 MiB of RAM instead of 1 GiB
- no on-board ethernet chipset

Add it as it is a closer match to what we model.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201024170127.3592182-10-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>