Document the following Raspberry Pi models:

 - raspi0               Raspberry Pi Zero (revision 1.2)
 - raspi1ap             Raspberry Pi A+ (revision 1.1)
 - raspi2b              Raspberry Pi 2B (revision 1.1)
 - raspi3ap             Raspberry Pi 3A+ (revision 1.0)
 - raspi3b              Raspberry Pi 3B (revision 1.2)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201120173953.2539469-3-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>