The Raspberry firmware is closed-source. While running it, it
accesses various I/O registers. Logging these accesses as UNIMP
(unimplemented) help to understand what the firmware is doing
(ideally we want it able to boot a Linux kernel).

Document various blocks we might use later.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Message-id: 20200921034729.432931-2-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>